Could you please be a little more specific in what you wish to accomplish?
I think you’re looking for the best models to “create” a city? Or are you looking for a modelling program to make your own structures?
A while back i think I saw a city generator script for Blender. I just found Suicidator (http://cgchan.com/suicidator), not sure if it is the same or not. There is both a limited free version and also a not so limited pay version, which is not that expensive.
Perhaps not as good as the esri CityEngine, but doesn’t have the high price tag either.