Personally I've found that once you go DIY, you don't go back. I always built the subs for my car when I was younger, but about 2 years ago I built my first HT sub, the Dayton 15 HO. I now have (3) SI 18" subs in my room and I'm looking for one more for a quad setup. Priced at just $179(plus shipping), they are a great value. You will need to build a box though and pick up an amp. Ultimately you're looking at like $500, but with room to grow to another sub on the amp at any time.
I highly recommend, if you feel comfortable enough, to build your own box with simple woodworking tools. Check out this forum for more info. You can post the same question and there are a lot of great people on there that will help
http://www.avsforum.com/forum/155-diy-speakers-subs/
If not, I'd look into the SVS subs.