barf...
Heres an idea, seriously:
http://tcsounds.com/db500.htm
For $150 these will stomp out any of them crappy subs posted so far.
Make a box 3 cubic feet sealed, feed them about 400-500 WRMS each and they will sing...
If you want to go even cheaper, Wal Mart has a 12: Durabrand sub on clearance for around 20-25$... I bought one today to test out, why not for $20?