If you decide to build your own sub, rather than buy one, then you might want to look at Outlaw Audio's M2200 monoblock amp, which sells direct (via Internet) for $325. For more info, go to this link: http://www.outlawaudio.com/products/2200.html
You may also want to consider just buying Outlaw Audio's powered subwoofer, which was designed for them by Hsu. It sells for $579, and has gotten some excellent reviews.