What camera do you own now? Does it have a manual mode?
You can get used DSLRs for $300, maybe even less. Look for Canon 20D's, 30D's, XT's, Rebel (300D rest of the world designation), Nikon D70, D50, D40, D80(probably a little bit more) or other DSLRs that are several generations old. It's not going to be the latest and greatest, but they're still more than capable.
20D w/ grip $300 OBO
Canon Digital Photography Forums
30D w/ grip $350
Canon Digital Photography Forums
30D ex condition < 5K shutter actuations $350 negotiable
Canon Digital Photography Forums
XT with 18-55 kit lens $245 shipped
Canon Digital Photography Forums
XT 7800 actuations $220
Canon Digital Photography Forums
XT w/ 18-55 kit $225 shipped
Canon Digital Photography Forums
300D w/ 30-85 f/4-5.6 $175 shipped
Canon Digital Photography Forums
I'm a member on only one Nikon board, so I don't have very good resources like POTN for Nikon gear. But with the above listings, you should be able to get the point.