Amazon has the Targus Urban Essential 15.6" Laptop Backpack (Gray, TBB59404GL) for $16 with free shipping on $25+ or with Prime.
Amazon has the Targus CityLite Pro Modern Compact Convertible Backpack for 12" to 15.6" Laptop (Grey, TSB937GL) for $31 with free shipping. Has tuck-away shoulder straps that convert backpack to briefcase.
Amazon has the Targus CityLite Pro Premium Convertible Backpack for 15.6" Laptop (Grey, TSB939GL) for $50 with free shipping. Has a patented DOME protection system, rigid, impact-resistant EVA-protected top pocket and tuck-away padded shoulder straps.