Campus111 via eBay offers this Military Tactical Backpack in six colors (Black pictured) for $22.68 with free shipping. That's $53 off list and the lowest total price we could find. It features nylon construction, mesh backing, adjustable belts and straps, and Molle loops. It measures approximately 18" x 10" x 9".
Of note, 13 Deals offers a similar backpack for $18.99 with free shipping.