For an inexpensive sub that performs well, I'd take a look at the
Dayton 12" Quatro Sub for $75. You'd only need a .7ft^3 box for sealed or 1.5ft^3 for ported with a 4" by 18" port. I'd probably go with the ported enclosure for deeper bass extension and better output with a relatively modest 120w.
There's also a
Dayton 15" Quatro for $85 which would need 1.5ft^3 - sealed or 3ft^3 - ported with a 4" by 17" port. It'll play deeper and louder than the 12" with that 120w at the expense of some trunk space.
I don't have any personal experience with the Quatro subs (although I've got a Dayton DVC for HT), but they've gotten great reviews on the PE tech board.
