It depends what you class as expensive. Stayed in Nottingham once and though it was cheaper, the hassle getting back wasn't worth it (but like anywhere with big crowds, one minute can make the difference between an easy and quick journey or a 3 hour traffic jam)
All the other times always stayed in Derby, last year was about 130 a night which to me is alright as the festival is my summer holiday.
Horses for courses really, but loads more buses go back to Derby than Nottingham and Leicester which also depends on if you plan to stay until the end.
So not much help really.