Here are just a few options in your price range...

Subwoofers:
15" Diamond Audio CM3, JL Audio 12W0, Alpine Type R, and Elemental Designs 13Kv.2.

Amps:
Soundstream Rubicon, JBL BP300.1, Rockford Fosgate P3001, and Alpine MRP-M350.

Throw in a $25 Wal-Mart wiring kit and either build your own box or get a cheapy pre-fab and you're way better off than any Xplod system.