I don't think they're should be limits on what someone can ask for but if the buyer is heavily penalised for going over a certain amount then eventually the selling clubs will reduce prices or better still keep the players and develop.
The 3 best teams in Europe over the last 3 years have transfer records of between £65m and 75m. If that's enough to win you the European Cup then surely that's an acceptable number to agree on and charge the so called luxury tax on anything beyond that? If an outside investor buys Newcastle and wants to buy Halaand then let him, he pays Dortmund the 200m and pays 130m extra in luxury taxes that goes into a fund to spread around European national associations. No restrictions against investment but it is heavily penalised if it gets out of hand.
Wages is a harder number to agree a starting point on but it needs sorting out.
If the transfer money stays within the game then generally that will still be good for football, the wealth gets spread. If some clubs get their transfer policy wrong then they risk losing their 'status', they would have to accept becoming Aldi rather than Waitrose. Boo hoo, that's business. If Haaland or Coutinho are a flop at the buying club then it's still a success for the selling club.
Money being taken out of the game is the real problem, by owners such as the Glaziers, by Agents getting ridiculous fees and in some cases by hyper inflated player wages.
The 'investment' by the new breed of billionaires and consortiums is generally all aimed at making significant money from the game in the short term. Taking it out of the game.
City's model is slightly different in that the owner isn't taking any money out of the game. OK in his case he's rich enough that he doesn't need to but maybe that's the place to start. Legislate tighter the areas where the money disappears out of the game.
Remember that after the initial injection of money into the club pre FFP that the rest of City's wealth has come from good management, strong commercial deals and success breeding success.
City didn't have the hyper inflated deals that PSG got away with either, the Etihad sponsorship was value for money. Not the lowest possible but not the highest possible either.
Allow competition. Allow cash injections into clubs but limit them e.g £x in 'y' years, and make sure that proof of funds exist first, possibly some form of football escrow.
If a club can attract a rich owner/investor willing to stake their
own money on building a successful football future then why shouldn't they be allowed to do it without others bleating that it's not fair.
Fair is a policy that gives any club the same opportunity of investment and building success, some will win, some will lose; that's business. Unfair is having rules that protect the established elite. Saying a Fulham or a Southampton can't invest money because they didn't have the turnover in previous years - which is what current FFP rules do.
FFP rules should be looking to the now and the future, not to the past.