It makes no sense to have a transfer cap. You're not gonna see great players move until they decide not to renew their contracts (which they probably won't do) since their club won't release them for the price being offered. And since they are so great, they'll already be in the top clubs (e.g. Messi in Barcelona) winning competitions, satisfied with where they are, unwilling to move to another club. I agree that the amount of money spent is ridiculous (namely Cristiano Ronaldo last summer) but what are you going to do about it?
To make matters worse, which currency are you going to use for your cap? The exchange rate fluctuates every day and it goes to multiple decimal places. UEFA is likely going to use the Euro, and if they set it at, say, EUR 80m before the summer transfer window, it's GBP 69,663,540.66 one day and if one currency crashes over autumn, it may end up being just GBP 52,523,536.89 (obvious exaggeration, I hope) in January.
Same goes for the salary cap. Even though most of Europe uses the Euro, two countries like Spain and Greece shouldn't have the same cap, right? If they do have the same cap, using Barcelona and Olympiacos as examples here, Olympiacos would obviously have a lot more room to work with, so if I'm Lionel Messi, I'd consider a move over to Olympiacos for the money.
It's just not going to work.