Check your local Craigslist for a late 80's/early 90's mountain bike made by Specialized, Trek or Giant. These things are great for commuting, and you can often find them in good shape for around $100. You might want to replace the tires, tubes, cables and brake pads to have it running like a new bike, but even if you have the LBS do the work it'll be cheaper than a low-end new bike. A lot of these can even be converted to single speed (depending on the dropouts). Avoid anything with suspension.