IMHO you can't beat Adire for the money....I love my Tempest and it wangs...and the SQ is phenominal, i'd look for some used ones and they will run you ~$100 or less, and you will be really happy, port it and be proud. Doesn't sport the XBL^2 motor like there newer subs but still has the underhung design, and its an awesome sub for the money, check them out at
www.adireaudio.com.
If you can fit a 15" that is, if not drop down to the Shiva line, same design less cone area, 12" that is.
The sub is really effecient too, ~ 400-500rms @ 4ohms and you'll be golden, and it'll sound awesome.
They used to retail for ~$150
They have a new design in progress right now, not sure how it'll fair compaired to the older designs.
I can't suggest any better for the money really. I've heard lots of good things about the new peerless lines, XXLS. IDMAX's are also nice.
As far as your actual price point i'm not sure, but for <200 i'd say tempest or shiva, over 200, i'd say Tulmult, Brahma, and something along the lines of a IDMAX or IDQ, or even a OZ Matrix Elite and ARC subs also. There's tons out there that arn't really mainstream that are so much better than the mainstream crap floating around. Research and listen and choose, not based on mainstream or who knows what company...but by the sound and price.