Groupon offers the Purafilter 2000 Gold Series Air Filter 4-Pack in several sizes (16" x 20" x 1" pictured), with prices starting at $22.99. Coupon code "SALE3" cuts that starting price to $20.69. With $3.99 for shipping, that's as much as $35 off and the lowest price we could find, although we saw them for around a buck less two weeks ago. Each filter features a MERV 11 rating and lasts up to 90 days. Deal ends May 20.
Note: This coupon can be used up to three times per customer.