How much will one of the European giants have to pay to sign Erling Haaland when the transfer window opens on July 1? Borussia Dortmund want in the region of €180m for the prolific striker, according to ESPN sources. If Borussia Dortmund keep him another season, then they could lose him for €75m, with his reported release clause kicking in next year.

Prospective clubs -- like Bayern Munich, Manchester City, Real Madrid and Chelsea -- have to weigh up whether to pay the premium this summer, or wait a season and then be involved in one of the biggest transfer scrambles in recent memory.

Haaland's potential is frightening, aged 20, he has already scored 20 Champions League goals, in just 14 matches. An unfortunate injury could derail his career; his goals record could dry up. But if he continues this trajectory, he will be one of the modern era's greatest players -- someone who could, along with PSG's Kylian Mbappe, capture the imagination of soccer fans as Lionel Messi and Cristiano Ronaldo have for nearly two decades.

And that's why those in the sports trading card industry were not surprised to see one of Haaland's autographed Topps "rookie" Bundesliga cards selling for an eye-watering $124,320 in an auction at the start of March.

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"There are so many things he can do in his career," David Leiner, GM global sports and entertainment at The Topps Company, tells ESPN. "I think the euphoria I've seen here is similar to what we saw with LeBron James 20 years ago in basketball. I remember back in 2003, 2004, 2005 thinking 'oh my! These LeBron cards are going for crazy money' and now you look back and think, 'actually, they were cheap.'

"With this Haaland card, we could be talking in five years and that card could be worth a million dollars. You just don't know where it's going to go."

Across all sports, the trading card market is seeing its records toppled on a near-monthly basis. For the vintage ones -- usually those cards produced before 1980 -- the record stands at $5.2m, paid by actor Rob Gough for a 1952 Topps Mickey Mantle card. For modern cards, an unknown buyer spent $4.6m in February for a 1/1 autographed card of NBA star Luka Doncic. But soccer is creeping up on the heels of the more established markets.

"Soccer is our highest growth category right now," Ken Goldin, founder of sports trading card specialists Goldin Auctions, tells ESPN. "When I started the business in 2012, the people looking for soccer cards were non-existent. It started becoming popular in 2019, more so 2020 and right now, it is to the point that if you took Tom Brady and Patrick Mahomes out of the American football market, soccer would be third for us, behind baseball and basketball. It is our biggest growing international market."

Sports trading cards vary in rarity, desirability and condition, and with different grades and different variations, it's a confusing market to understand from scratch.

If you look at a trading card auction, you'll see a number of letters and numbers alongside the player name for each card. There are respected rating companies, like Professional Sports Authenticator (PSA), who are capable of "grading" the condition of every card. If they give a card a 10, it's perfect and unblemished -- the best version of that card one could find. From there, it's a sliding scale down to 1 based on everything from the state of the card's edges, to fingerprints or smudges.

All trading cards are originally released in sealed packets, sold at newsagents or specialist shops. Each athlete typically has a "base" card -- their most common one in any edition -- and some players are then included as a "Chrome" edition (simply a classier, metallic-looking premium card). Then there are "refractors" (the same cards, but in different colours) and "parallels" (numbered versions) of the player card. The fewer the numbered versions, the rarer it is. The next level up is even rarer autographed cards, or others with pieces of game-used jerseys embedded in the cards.

But then there is the SuperFractor -- the Mona Lisa of the sports trading card market. It's a special edition refractor card, with just one in existence, and randomly seeded into packs just like any other card. Unearthing that would be like winning the lottery.

"I have no doubt when that 1-of-1 Haaland card is pulled, that's going to be a million-dollar card," Leiner says. "You are going to see a million-dollar football card out there."

Mbappe and Haaland are the new kids on the block for soccer collectors, while Goldin says demand is skyrocketing for early rookie cards (largely because fewer were printed) of Messi in his early years at Barcelona, or Ronaldo from his Sporting Lisbon days. It's hard to compare prices here between the Haaland one that sold and these two icons as there were never any autographed rookie cards of Ronaldo or Messi, but a standard Messi rookie card graded 9.5 from the 2004-05 Panini release will set you back in the region of $100,000.

The Haaland card that sold for over $120,000 isn't quite the Mona Lisa, but would equate to one of Leonardo da Vinci's more recognizable works. There are just 25 in existence, and the one that sold is numbered 1 of 25: considered the most desirable. It is from the 2019 Topps Chrome Bundesliga catalogue, and is an autographed orange refractor, graded a mint 10 by PSA, from Haaland's first season with Borussia Dortmund -- essentially his 'rookie' card in the Bundesliga. Better yet, it's also signed by the Norwegian striker (it's rarer for hot prospects to autograph cards).

"Haaland is a goal scoring machine," Aaron Nowak, CEO of sports trading card website SlabStox, tells ESPN. "He's super-young, one of the highest valued prospects in the market right now. I'd say Mbappe was the most coveted prospect for the last year and a half, but now Haaland has risen to that level.

"There are a lot less of these soccer cards made than basketball cards because the market was smaller. The amount of demand in 2014 to 2019 was a lot smaller than today, which led to this massive price increase because there are so few cards to be had. Some of the most basic Haaland rookie cards are going for $4,000 because there are just 450 PSA 10 graded in circulation. Compare this to Doncic, where there are 18,000 base PSA 10s. That specific Haaland card hit the auction block at the perfect time."

The record-breaking card was sold by Goldin Auctions, one of the highest profile auction houses for trading cards, in early March.

"I was shocked when it sold for that," Goldin says. "I am a traditional American sports guy and to be honest, when my consignment director brought it in to me saying he was excited by this, I honestly asked him who that was. And then I quickly learned... I knew Messi, Diego Maradona, Ronaldo, Pele, but I did not know who [Haaland] was. I went from not knowing who it was to becoming his biggest cheerleader and biggest fan, and looking for everything that we could get our hands on that's related to him."

Goldin remembers the auction well, as the price tripled in the final 24 hours.

"It's a one-year-old card, produced in 2019. This puts soccer on equal footing with baseball and basketball in terms of price level, the demand and the importance of its superstar players." Online auction house eBay has also seen a spike in interest: global trading card sales were up 142% in 2020 on previous year, with over four million cards sold. Within that, soccer cards experienced the biggest spike.

This Haaland card plays into the larger narrative of the trading card industry and its remarkable boom in general interest. Those close to the industry claim it was in part accelerated by COVID-19 lockdowns, as well as the subsequent time spent at home digging out old hobbies. But there's also the growing demand for soccer memorabilia in the U.S., thanks to the increased exposure of the sport on linear and direct-to-consumer (DTC) platforms, and the increased stature of the U.S. internationals and North American players shining in the Champions League, like Bayern Munich's Alphonso Davies and Borussia Dortmund's Gio Reyna.

"For the past five years or so, trading cards have increasing in double digits. [The year] 2020 was an acceleration of that growth, and we hit numbers we thought we might hit in 2022 or 2023," Leiner says. "When it comes to football collectables, this to me was like the most obvious secret around. The Haaland card will definitely drive interest and intrigue: it draws attention to the hobby and how exciting it is to get one of these Willy Wonka golden tickets."

For those in the memorabilia and trading card market, this puts Haaland in the "prospecting" bracket. Investors are gambling on his future ability by buying desirable rare items now, with the hope their values will increase alongside his stature. The focus for those who dabble in the "prospecting" markets are Haaland and Mbappe cards, but other players like Man City's Phil Foden, Real Madrid's Vinicius Junior, Reyna, Bayern Munich and Canada defender Alphonso Davies, Man United forward Mason Greenwood, Atletico Madrid's Joao Felix and Barcelona's Ansu Fati are also in demand. Says Goldin: "They're viewing [Haaland] as the superstar for the next 15 years and that's what drives my marketplace -- it's potential. You're almost betting on the athlete's future: [the sale of the Haaland card] is a really big bet on his future."

The old faithfuls like Cristiano Ronaldo and Messi are also selling like hot cakes to the extent that buyers are purchasing unopened boxes of Panini's 2002-03 "Mega Craques" range, originally sold in Portugal, for up to $88,000 off Goldin Auctions, in the hope of unearthing a mint condition Ronaldo rookie card. (One sold in January for $205,000). "It's better odds than the lottery," Goldin says. Then there are the expensive vintage cards like Pele or Maradona, which sell for the price of a small Vincent van Gogh watercolour: one of only five mint condition Pele cards graded a 9 from the 1958 "Alifabolaget" set sold via Goldin Auctions for $288,000 in November.

So the big question for those looking to get into the market, or having their interest piqued by this Haaland sale, is simple: when is the right time to buy and sell, and has this market reached its limit? "A high tide raises all ships," Nowak says, pointing to a specific Mbappe card that went from $6,000 to $8,500 off the back of his Champions League hat-trick against Barcelona in February. Goldin says this market is in the "infancy," and advised one buyer who had a $5m budget to invest 40 percent of that into soccer trading cards.

For Leiner and others in the industry, they see physical trading cards as being able to complement the demand for digital trading cards. The rise and blockchain purchases of non-fungible tokens (NFTs) in the digital sphere -- like the NBA's Top Shot phenomenon, whereby fans are buying authenticated highlights for prices in the six-figure range -- has seen digital trading cards emerge as a valuable commodity. A chrome Cristiano Ronaldo digital trading card sold for $289,920 on the platform Sorare in mid-March.

Topps GM Leiner says the company are already deep in investing and developing the digital card market. "We think the prices are going to come down to earth a little bit here, but it's going to be a strong market and it's something we see as a long-term play; we will definitely be doing more NFTs soon."

"With the European Championships just around the corner, the World Cup next year and then the 2026 World Cup in the USA on the horizon, we're still very much in the 'boom' stage of this growth," says Nowak. "We've seen that with Luka [Doncic] and Zion [Williamson] this year and that really helps propel the market further." At Topps, they are preparing to release the latest batch of Bundesliga cards, updated versions potentially including some cards with match worn kit, later this year. They expect the boxes to sell out on their website within three seconds.

The record-setting Haaland card may well reappear for auction in the future, but for now, the search goes on for the Haaland Holy Grail: that SuperFractor card from the 2019 set. As long as Haaland continues this remarkable trajectory and continues to bang in the goals, the price of his cards will increase.

"Right now, especially after this sale, [Haaland's] probably number one everybody's looking for," Goldin says.