--cut--
It's so sad that even a big team like Barcelona can't even afford to sign Nico Williams, how about Mbappe? it's impossible for them to sign Mbappe, they might only able to buy Mbappe's hair lol.

Yes, this is very sad, I must say, I also think that something is broken.
The problem is also the investments made with Xavi which have not returned and consequently the team is in a greater crisis.
Certainly the salaries and costs of the players influence the crisis of modern teams.
Poor management does further damage. In my opinion..
FIFA? You meant UEFA, right? La Liga has Financial Fair Play, and UEFA has FSR rules. These rules already say how much a team can spend in a season. A La Liga team can spend 50% of its revenue on player transfers. UEFA set their limit at 70% for a team can spend it on player and coach wages, transfers, and agent fees.
But the main problem still Barcelona itself. Barcelona has bad financial management. It was the main reason they struggled. This is because Barcelona never try to be efficient with player transfers. They keep recording more and more losses on their balance sheet.
It's impossible to limit the player's price. This is because it depends on the club that owns the player's contract. How can UEFA value the importance of player for the club, without the club itself who can value their player?