You could pick up a 12" Aluminum Cone Eclipse on Ebay for around $185 + shipping and a JBL BP600.1 for $175 + shipping. Build your own box, and for around $450 you could have a very nice sounding sub with a decent amp to push it. Many people would also suggest Lambda, although I can not testify to how they sound; it will take close to a month before they ship it out to you (thats what changed my mind).
