Amazon offers the 3M Filtrete Ultra Allergen Reduction 12" x 24" x 1" Filter 6-Pack, model no. 2020DC-6, for $77.02. Clip the $18 off coupon (found directly under the product price) to cut it to $59.02. With free shipping, that's $9.84 per filter and $17 under the lowest total price we could find for this quantity elsewhere. These air filters last up to three months.
Note that this item will ship in three to four weeks.