LightInTheBox offers the Makino 40-Liter Water Resistant Backpack in Dark Grey, Blue, or Fuchsia for $39.99 with free shipping. That's $40 off list and the lowest price we could find. It features nylon constriction, adjustable straps, and it measures approximately 6" x 16" x 24".
Note: This item ships from China and may take two to five weeks to arrive. Uncheck "Add Shipping Insurance to your order" during checkout to remove the 49-cent fee.