Deal Genius offers the 3M Wet Or Dry Fine Automotive Sandpaper 15-Pack in 400 Grit or 2,000 Grit for $6.99 with free shipping. Except for the mention below, that's the lowest total price we could find by $6. Each sheet measures 3.67" x 9".

Note: Walmart has 5-Packs for $2.36 with in-store pickup (15 for $7.08).