Dzeko is a better player, he's clearly quality but at the end of the day, Dzeko will also be on nearly double wages than Bent and in terms of importance to the team, Bent will be more important to Villa than Dzeko at City I would imagine so in terms of fee's, I've personally got no problem, Bent has now the same amount of goals as Rooney and Drogba so when you buy a player who can score so many goal, almost guarenteed...it's going to be worth the money.
Would Bent get into City? No, but Bent is a quality proven Premiership striker and that costs money. You buy a goalscorer like that somewhere out the Premiership and he costs £4-5 million less but they'd of never played in the Prem before and it's not a guarentee that they will perform and adapt, hence the larger fee.