Amazon has the 4-Pack Filtrete 24 x 24 MERV 11 / MPR 1000 AC Furnace Air Filter on sale for $33 - 15% off coupon (check "Save an extra" box, eligible on first S&S order for this product) - 5% off with Subscribe & Save (can cancel anytime) = $26 with free shipping on $35+ or with Prime. $6.60 per filter.