Amazon offers Prime members the uBoxes Queen Mattress Poly Cover for $5.99 with free shipping. That's the lowest price we could find by $6. This 2-mm heavy-duty polyethylene cover measures 61" x 15" x 90" and slips over your mattress like a pillowcase. Update: The price has dropped to $5.39.

Need to buy in bulk? Jet offers a 12-Pack for $31.56 $36.53 plus $5.99 for shipping. (You can also spend over $35 to bag free shipping.) Even with shipping, that's $3.13 per cover and the lowest price we could find for this quantity by $34.