Mwave offers the Boss Audio 12" 700-watt Car Subwoofer, model no. CX122, for $34 with free shipping. (Amazon charges the same with free shipping for Prime members.) That's the lowest total price we could find by $3. It features 700-watt RMS, 1,400-watt peak output, 4-3/4" mounting depth, and 30Hz to 3kHz frequency response.
Update: Specs corrected