How far is your commute? A mountain bike can be just fine for commuting. If you're going to ride on the road, you don't need a bike with suspension, so you could get a "hardtail" mountain bike (without rear suspension) and no front suspension (or lock out the front suspension). Put slick tires on it for the road, and you'll have a decent bike in your price range.

You could probably save money, though, by buying a used bike. If Craigslist and classified ads are not turning anything up, you could check out yard sales, which often have bikes. You could also let the bike shops in your area know that you're looking for a used bike, so that if a customer your size wants to trade in their old bike, the shop can contact you. It looks like Freeze Thaw sells used bikes. You could also put a "wanted" ad on Craigslist, specifying the kind of bike and size that you would want.

If you decide to go the yard sale route, you should read up on bikes so that you can get a good sense of what distinguishes a quality bike from a department store bike. A couple of useful diagnostics: cheap bikes often have steel rims rather than aluminum (use a magnet to test), and they often have one-piece cranks. A bike that has Shimano or Tektro components is likely to be decent even if it's low-end. Look too at the quality of the welds on the frame; they should be even, with no gaps or irregularities. A rusty chain is fine as long as the chainrings and sprockets are in good shape. If you need help with the terminology, visit Sheldon Brown's bicycle glossary.

Good luck!