Klipsch KW-100 10" 200 Watt Powered Down-Firing Subwoofer for $148.00.  Shipping is $24.36 for a total of $172.36 shipped.  Here is a screenshot for your reference.  It features a 10" down-firing driver, 29Hz-120Hz frequency response range, 40Hz-120Hz low pass crossover, and more.  Most places have it for about $299.