Klipsch SW-350 8" Subwoofer for $159.99. Shipping is $5.00 for a total of $164.99 shipped.  Here is a screenshot for your reference.  It features an 8" fiber-composite down firing woofer, 31 to 120Hz frequency response, Class D amplifier, and Bass-reflex enclosure. Most places have this subwoofer for about $295.