How cheap you talkin' about? Get a used JBL BP600.1 and a 10" TypeR, build a ported box. About $200 plus box and wiring, and should sound very nice. A 12" isn't much more.
http://www.etronics.com/product.asp?stk_code=alpswr1042d&store=&catid=368
http://www.etronics.com/product.asp?stk_code=alpswr1222d&store=&catid=369
I've also bought from
www.IkeSound.com but their shipping charges are usually $20. So the 10" there is about $10 cheaper than IkeSound.
Check out
www.carstereos.org and see what they have for used audio. I bought several things from there.
