A thought...
Check eBay for packs. I was lucky to pickup a small Duluth Pack - I think they call it a European style rucksack - for about $30 a couple years ago. I use it exactly for the purpose you're talking about. I was using one of my son's old Jansport bookpacks, and although it was good it isn't near as durable as the canvas Duluth. But brand new, well, they're mighty pricey.

Good luck,
Wind-in-Face