Here's my entry.
1989 Ninja 250. I paid $450 for it as it hadn't run for over 2 years. A new battery, a few misc parts, and a tune-up and I've got about $900 total into it. It has 12,000 miles and runs great other than being really, really cold blooded. In 2 weeks when my son goes through his motorcycle safety course, it will be his. Should be the perfect bike for him to tear around and learn on.