Welcome to the herd!
Seeing that you are in a large college town my first recommendation would have been Craigslist, but it seems you have already found that one. With classes starting up there should be lot's of bikes for sale. The same will be true for the end of each semester.
If your roommate's Rockhopper will fit and the price is right, that is your best solution. Clean up on a bike with some rust is not that hard, and it's cheap. Keep in mind if it's been sitting you may need some overdue preventative maintenance and probably some street tires unless you are going to hit the trails.
EDIT: Just realized the possible down side to your roomies bike being a RockHopper. Does it have a suspension front fork? If it does, does it have a lock out on it?
Last edited by txvintage; 08-22-09 at 01:15 PM.