Newegg offers the Ranipak Luggage Durable Utility Laptop Backpack in Black, model no. B0158-BK, for $24.99. Coupon code "EMCAKPA72" cuts it to $14.99. With free shipping, that's the lowest total price we could find by $40. It accommodates a 15.6" notebook and features an iPad/Tablet compartment, smartphone organizer, and water bottle pocket.
Note: It's available in five additional colors for $24.99. That's a price low by $30.