It's tough to find a good cyclocross bike, even used, for $500. It also might be a little tough to find a good cyclocross bike for someone your size. Bikesdirect.com has some cross bikes in that price range, but determining whether or not their sizes are good for you could be a problem.
I mentioned in another thread that I bought an earlier 90's Specialized Hardrock for my daughter so she could try cyclocross racing. She's about 4'10" and the 13.5" model I got is just a little big. This bike was originally marketed as a mountain bike, but it's got a rigid fork and cantilever brakes. A good bike co-op could help you put drop bars on a bike like this and it would be a great commuter and a decent bike for trying out 'cross racing.