Campus111 via eBay offers the Eaglewill 20-Liter Hydration-Compatible Backpack in Black or Green for $13.49 with free shipping. That's the lowest price we could find by $7. This water-resistant backpack accommodates hydration bladders up to two liters (sold separately) and features a large main compartment, two front pockets, bottom mesh bag, and adjustable padded shoulder and chest straps.