You're in a bit of a tricky spot with that budget and experience level. Anything you can buy new isn't worth buying (for the reasons Aubergine gave you), but anything used would require decent bike knowledge to assess and/or to repair into good rideable condition. Do you have a bike co-op or similar where you're located? How about a friend who knows bikes who could help you browse through Craigslist?
In your shoes I would be looking along the lines of what Aubergine suggested, probably trying to keep the budget to ~$100 so that you can spend another $100 for a tune-up and parts replacement.