Amazon offers Prime members the Dakine Jewel 26-Liter Laptop Backpack in Kona for $34.97 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $5.58.) That's the lowest price we could find by $6, excluding the mention below. It features an organizer pocket, side mesh pockets, and will fit most laptops up to 15".
Note, this item may be on backorder but can still be purchased at this price.
A close price: eBags via eBay charges $35.99 with free shipping