Coupons.com has a $3.00 off coupon for Filtrete Air Filters with a 1000 or higher MPR rating. These are not the cheapest air filters but not anywhere near the most expensive. These go for $12.88 at Walmart so this brings them to under $10.00.  The better air filters are worth it for your AC or heating units.

Filtrete 1200 MPR Air Filter: $12.88
Print $3.00 off Coupon
Total: $11.38