If you are buying new, a good subwoofer that will be relatively easy to integrate into your system (which requires, as others have noted, that the subwoofer have some sort of automatic room correction software) will run between $2400 and $3200, minimum -- cost goes up from there for bigger drivers and a little deeper response. Good brands to look at include JL Audio, Velodyne, and REL. I would caution against buying an off-brand without auditioning it first -- a lot of them are just glorified boom boxes.
