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
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