By this time, (well after school has started) Goodwill should start having some back packs again, generally for under $5.00. I just picked up a carry-on size suitcase in good condition for $3.99...
If she is midlde school or older, I would invest in a lands end. It will be the last one you have to buy. I got tired of having to buy half way through the year and not finding one. This lands end one have lasted live 3 or 4 year and is still perfect!
I LOVE LL Bean backpacks. They aren't the cheapest ones out there, but they last a long time, and you can send them back when they wear out. My oldest daughter had hers all 4 years of high school and is now using it her first year of college. My youngest has had hers for 2 years (this is the 3rd) and the handle is starting to rip so we are are sending it back Friday for a replacement!