Adorama offers the Polk Audio PSW111 150-watt 8" Compact Powered Subwoofer, model no. AM1145-C|4, for $134.99 with free shipping. That's the lowest total price we could find by $25 and the best deal we've seen for this model. It features a frequency response of 38Hz to 250Hz, 8" driver, and 60Hz to 120Hz variable crossover.

Update: The price has dropped to $124.99.