I think he's easily still a £20-25m player. Inconsistent as hell, but on a team where he played every game he'd score 30 goals in a season in all likelihood. I think City would probably take at an absolute minimum though something like 20m. He is still under contract until 2015 so there isn't a need to sell although I expect it will happen.
Didn't City pay about 25m for him? I'd say that he lost value rather than gained value, and since City is a wealthy club doesn't need to sell players, that weakens their bargaining position further.
He is still young, and it's clear he's technically talented, but I don't see a player like him going for above 15m. It's clear that he's a slight "risk" in terms of his personality - compare him to someone like Van Persie, who despite planning to leave Arsenal, gave his absolute all in every game and was a top, top striker, then moved to United and jumped in with both feet, giving 120% in every game. Dzeko just seems like he couldn't give two shits about Manchester City, so long as they aren't serving Edin Dzeko. Any future club risks that they have a disinterested player who goes on tilt easily.
Of course, as it was pointed out Balotelli did go for around 20m. But an Italian going to Italy does change things a bit.