Its gonna be a difficult thing to negotiate with the PFA at this point. To get salary caps instituted in US sports, they basically traded the players free agency. In this case, the Bosman ruling has already given players that right...so what can you bargain with?
Most effective tactic (what the NFL Ownership just used in the latest CBA) is to target the PFA's weakness - the vast majority of its membership, enough to approve any agreement, will never reach the heights of those massive salaries, and thus don't really care much about what the "cap" is. What do they want?
The trouble football faces is its not a single league, and there will be leagues that exploit the players that want to leave to maximize their salaries. Are we prepared to have a league with no Kanes?
German football with 50+1 and its lower prices and what not is being idolized currently. But the realities of the league is also that, outside of Bayern who operate on a different financial level, any time a star develops in the league they generally very quickly either move to Bayern or switch leagues.
So can you sell this UEFA wide? First you have to get the players to form an international union to negotiate with, yeah?