City spent a fair bit but it wasn't out of sync with thr market, adebayor for 25m is comparable to drogba going to Chelsea, Drogba a better player but adebayor signed from a direct rival. Aguero was considered a generational talent and signed for similar amounts to what we got Rooney for but a decade later. It was when they started signing players that were somewhat average. 40m from Fernandinho, not average by any means but coming from the Ukrainian league, then mandala the following season for 45m from Portugal. Its things like this that started sending the market crazy, they started paying the "feck off" prices that clubs give when they want to hold onto a player. Normally the player would stay an extra season or two til the contract ran down, as Fernandinho and mandala would have done, but City started paying these obscene prices teams would quote and it radically shifted the market.
Psg did more to affect it, but City were increasing it, the same way Chelsea did from 04-06 they never quite broke what we spent in ferdinand or Rooney, but they started making 20m and 25m transfers commonplace. Its also not just how it affects the market, there's also the Andy Carroll effect, teams know the other team has money because they've just sold a player for big bucks, so they get charged more for the replacement than they ordinarily would