Soccer has always been the world’s favorite sport, but for most of its history, even the best players were not particularly well paid, and the stories of former greats who struggled financially after retiring are often hard to contemplate for modern soccer fans.

However, the days when soccer players were essentially low-paid employees have long gone. Since the world’s first million pound transfer in 1975, wages and fees have increased dramatically. Cristiano Ronaldo’s £80 million move from Manchester United to Real Madrid in 2009 grabbed the headlines at the time, but that total has since been exceeded six times.

Of course, it would be hard to argue that Ronaldo wasn’t worth the money. At Madrid, the Portuguese star has helped the team to win two La Liga titles, two Spanish Cups, and three Champions League trophies, helping them to stay at the top of Spanish soccer and remain among the favorites on Stakers.com for any competition in which they take part.

He has also helped the club to sell millions of shirts and Ronaldo-related merchandise, and you could argue that he has earned that £80 million back for the club in the past nine years. Whether the same applies to his teammate Gareth Bale seems less clear. Bale followed Ronaldo from England to Spain in 2013 for a new record of £86 million, and while he has had his moments at the Bernabéu, injury has prevented him from consistently being at his best.

Bale’s record lasted for three years, until it was broken by Manchester United’s signing of French midfield star Paul Pogba from Juventus. However, in two seasons at Old Trafford, Pogba has not lived up to his billing, and there is even talk that he might leave United this summer.

A year later, the record was shattered again by the enormous £198 million fee that took Neymar from Barcelona to Paris Saint-Germain. However, as with Bale and Pogba, the Parisian fans have yet to see the best of their expensive purchase, though the talented Brazilian has time to find his feet in Paris and could yet repay some of that fee if he can help the big-spending French club to progress in Europe next season.

Since Neymar’s move, there have been two other £100 million transfers, both in January this year. Philippe Coutinho moved from Liverpool to Barcelona for £105 million, while Kylian Mbappé made the switch from Monaco to PSG for a staggering £128 million.

If transfer fees continue to rise at this rate, could we one day see the world’s first billion dollar soccer player? It isn’t as unlikely as it sounds, and not surprisingly, many in soccer are worried. It may be arguable that the likes of Ronaldo are worth their fee, but the knock-on effect of escalating transfer costs is that players lower down the scale are also demanding higher wages and commanding ever-rising fees. This may prove to be unsustainable.

A salary cap, of the kind that exists in most US sports, may be one way to ensure that soccer doesn’t end up pricing most clubs out of the transfer market, but it would be hard to enforce and is unlikely to be supported by the big clubs. For the foreseeable future, the soccer transfer market will continue to escalate, and fans all over the world will find themselves asking the same question of the next multi-million dollar purchase: is he worth it?