you have to account taxes. you are not just paying 3 m a year in wages to a player, you are paying 6 million.
that can easily add up when you start getting ambitious and want to lure good, established players in.
Not to mention, players that start getting impatient and not winning and have the biggest clubs in Europe knocking at the door interested in the players, it begins to unsettle them.
If, forwhatever reason, they were able to attract some star players, it's REALLY going to cost them in wages, let me explain:
If I was a WC player, that could walk into any club in the world and had to choose between, let's say Palermo and Barcelona

D), I would rather take a 6m contract and go to Barca with the chances of actually winning a title MUCH MUCH higher over an 8m contract for Palermo with MUCH MUCH lower chances of winning a title. I'd still be getting a great wage BUT with the guarantee of competing for 1-2 titles a year, while Palermo are just "trying" to win the league.
It would take HUGE contract offer to convince a player to play in Palermo with no guarantees.
Not to mention, clubs like RM or Barca or United have incentive bonuses that they can shell out to their players for winning trophies, something Palermo can't do...even with 100-200m investmented.