I know the timing of this response is bad, but try looking at discount sellers near the beginning of the school term. I picked up an EDC bag (for toting things to work, not for a BOB) for $15, which retailed at $60.

If you're just looking for a very inexpensive back pack with better straps, I'm sure those $5 cheap school packs I see at some places would ride at least a little better than a drawstring bag.