Don't they have limits on how much they can spend ?
I'm sure they do. With oil rich billionaires the limits are beyond comprehension. If you're talking regulation, Clubs in Europe are supposed to abide by the UEFA Financial Fair Play rules. In theory this was supposed to help prevent clubs from going crazy, overspending and going bankrupt. In practice, it has strengthened the top, established club's stranglehold on the world game by limiting the ability of near-top-level clubs from “buying their way" on to the top table.
So is there a limit? No. You could spend €1 billion on a player, and pay him €1m per week if you were so inclined and you had an investor willing to spend it. But if your club didn't net-off that fee and wage through other revenue, you'd end up on the receiving end of some ire from the governing body. Assuming you hadn't already bankrupted the club in the meantime