Reply to Budget Subwoofer For Mobile Setup

I wasn't sure where to put this. But here it is, I'm looking to get a mobile sub this summer, and I'm on a budget, looking to spend around 500 bucks ish. I was also believeing of building a sub box and all, since I build them for sub in my cars, and friend's cars. And I've seen good deals on Crown and QSC amps on ebay and some local places. I was believeing if I build it to do a 15 inch woofer at around 800 watts. I want to use it in medium ish basments, and rooms. Do you guys have have any suggestions on this?
thanks.