I'd check your local bike store (LBS) for good used bike deals. I know the one by my campus does pretty good business, plus if anything breaks, you can take it there for some (hopefully) sound advice and fairly priced repairs. I see a lot of students riding around on the WalMart type bikes - and hey, if that works, great - but honestly, I doubt they cost much less than a used bike from your LBS and I doubt you'd feel confident in riding them.