The market is only inflated when players are in demand and the selling club doesn't want to lose them.
Hart, because of his age and wages, and because City actively want rid, isn't going to be in huge demand in terms of the actual transfer fee. It'd likely be a low fee to whoever is willing to pay him the money, or if City can subsidise his wages. Same story with them shifting Nasri too.
I'd be very surprised if Hart went for anywhere near £15 million.