In Columbia, I've heard good things about Klunk Bicycles (but never been myself). Pretty sure they have a used selection that turns over pretty quickly, though I have no idea on their prices. It is the end of the school year, so if you are creative and have no shame about digging through trash/checking with University security for abandoned bikes/etc I'd guess you could find something free or cheap a lot easier than those of us in bigger, non-college towns.

Craigslist might work for you, but pickings can be pretty slim there. Also, don't forget to budget for a good lock, helmet, lights, etc. Going cheap on that stuff you could get away with spending $60 or so, I think. You could also try posting a bike wanted ad somewhere at school, and pick up something a student headed home for the summer may have just abandoned, along with any necessary accessories.

In other words, if you try hard enough, I think you can find something that fits your budget. Just to tool around Columbia/campus, your biggest priority besides cost should be fit, doesn't matter if it's hybrid, road, whatever. Of course, finding something light enough for you to easily lift is an entire other problem. Just think of it as a cheap workout. And if you start wanting something lighter/faster/slower/shinier/sparklier/whatever, you'll have a much better understanding of what it is that you want, and hopefully be able to save a little cash to get it. Good luck!