If he was a really top notch player I could understand it, but he isn't that good, so £28m is a joke, no matter how much money you have. If you go to a Mercedes dealer and offer them £200,000 for a £80,000 car it doesn't mean it is a good purchase just because it is a quality car and you can afford it, it means you have no idea about value.
Silva I think was closer to a fair price, but they only got him for that because of Valencia's financial problems, otherwise they could have turned that down flat. The same for Villa to Barcelona. I don't remember even Chelsea paying prices that stupid for players.
I think he will probably do well as he is a good player, more suited to English football, at times at Barcelona he looked like a bulldog trying to run with greyhounds.