Groupon offers the Purafilter 2000 Gold Series Air Filter 4-Pack in several sizes (16" x 20" x 1" pictured), with prices starting at $19.99 plus $3.99 for shipping. (Most sizes are $19.99.) That's as much as $36 off and the lowest price we could find. Each filter features a MERV 11 rating and lasts up to 90 days. Deal ends May 6.