In reply to Itzal:
I picked up a Regatta Landtrekka (now Landtrek) 25L in a charity shop some years ago for £2.50 It takes a laptop and a couple of files no problem. It has a reinforced back. I'm not saying you'd ever find such a one, but I see an updated version for sale in "Outdoor World Shop" for £12.99 - worth a look?