eBags via eBay takes up to 75% off a selection of laptop and messenger bags, with prices starting from $9.34. (Although the banner states up to 50% off, we found deeper discounts within. Prices are as marked.) Plus, most items bag free shipping. A couple of best bets:
eBags via eBay offers the SwissGear Travel Gear 15" Laptop Backpack, model no. 3261, for $39.99 with free shipping. That's the lowest total price we could find by $4. It features a 15" padded laptop compartment, padded tablet pocket, organizer pocket, and compression straps.