I think, with no evidence whatsoever to back this up, that in the early stages players like Aguero etc would have been paid a lot more than would have been reported.
Also, they seem to have lucked out in terms of the personalities of players they got. Aguero, Silva etc never really seemed like problematic personalities or players that would have agitated to go elsewhere. Yaya was and he was frozen out and eventually left after some time. Sane, for example, was one of the best wingers in the league but he left after a relatively short period of time. I think it's a mix of the money, and getting lucky with the right set of players.