Diamond Audio CM3 12''
This is a pretty good sub...if you are looking for SQ. It doesn't take that much power to run, and can be bought for a little over $100 @ onlinecarstereo.com .
This is my second 12'' sub, the one I had previously was a JL12W0. The Diamond Audio, in my opinion, is better than the JL12W0 in every way, and it costs a little less (depending on where you shop).
If you are looking for a good sound quality sub, this is one to consider, I have heard good things about the other Diamond Audio subs (higher end), but I have not been able to hear them yet.