I'd guess the best bang for buck in that category might be the Hsu STF-1 for $299. Some say good things about the 10" TITANIC MKIII kit from Parts express ($349).
There's no need to timbre-match a sub with main speakers, so there's no reason to buy the same brand.
I personally like my B&W ASW 750 SUb. I agree with above HSU is good and excellent for home home theater set up. B&W is a Music type of sub--for HT I have to work it hard.