Your recent top purchases have been either by activating the release clause or in Lewandowski's case, him running down his contract. Had either of the 3 not been such cases and you wanted them, you'd pay considerably more than 30mil. Not that I'm trying to compare the likes of Lewa, Gotze or Thiago with someone like Bony but there's a reason why Bayern havent had to pay top dollar in recent times. Wasnt Benatia too bought with just a year left on his contract?
Anyways, my point is that in City's case, clubs know they have the money to spend and when you want a club like Swansea to sell their best player mid way through the season with no chance of replacing him with anyone even of similar quality, you have to pay up. Even in the summer, a player like Bony will cost 20mil easily and if 10mil gives him 6 extra months to integrate and also gives a massive boost to their title challenge considering how injury prone Aguero is, I dont see why a 10mil premium is such a big deal. Unless they could find such a striker with a year or so left on his contract, they had to pay up. Its the price you pay when you want another club's best player mid way through the season.