I buy my Roombas from Woot.com.  They're refurbs and older models, but $100 is about all I'm willing to spend on them.  When my first battery died I just bought another Roomba since for $100 it's nice to have another set of brushes, rollers, bearings, motors, etc.
I like my Scooba even more.  Other than tight corners, that thing does a great job.