I have to largely agree with Kevin. There are any number of cars you could drop safety equipment into in a matter of days and go run lemons safely. There are probably half a dozen on craigslist right now. Even if you have to have the cage custom fabbed, that doesn't count against the $500, so that is even doable.
Here are some candidates from a 60 second search.
http://reno.craigslist.org/cto/1093024632.html
http://reno.craigslist.org/cto/1093489981.html
http://reno.craigslist.org/cto/1093858951.html
http://reno.craigslist.org/cto/1093902240.html