Well for a budget of 500 dollars and you have nothing ur not going to be able to get that much stuff for ur ride u need wiring, a HU, sub and sub amp and anything half way decent is gonna cost more than $500. A good HU and the all the wiring, fuses, rcas, and such is gonna be about half of ur budget alone.
Realisticaly for a few more hundred i could see you getting it all if you know what to look for and get things cheap, my suggestion would be to look in the for sale thread and see what people have to offer.