LightInTheBox offers this Outdoor Sports 6-Liter Backpack in seven colors (Army Green pictured) for $14.99. Coupon code "SO5569221" drops it to $9. With free shipping, that's $39 off list and the lowest total price we could find. It features adjustable shoulder straps, zippered side pockets, and measures 12" x 7" x 3".
Note: Uncheck "Add Shipping Insurance to your order" during checkout to remove the 49-cent fee.
Update: Specs corrected.