Ending today, 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. That's tied with our expired mention from two days ago, 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.