eBags via eBay offers the Samsonite Modern Utility GT Laptop Backpack in Navy for $49.99 with free shipping. (Best Buy via eBay charges the same.) That's $50 off and the lowest price we could find. It fits most laptops up to 15.6" and has a water-resistant design.

Need it for Christmas? If you order via Best Buy direct, select stores have same-day pickup available.

Update: The price has increased at eBags; however, Best Buy via eBay and Best Buy direct still offer it for the same price.