Today only, for its My Best Buy members, Best Buy offers the Samsonite Tectonic PFT 17.3" Laptop Backpack in Black/Red, model no. 51531-1073, for $39.99 with free shipping. (Not a member? It's free to sign up.) That's the lowest total price we could find by $20. It fits laptops up to 17.3", and features a front pocket with organizer, oversized top pocket, and adjustable shoulder straps.
Update: It's now available at this price for everyone through July 23.