I would say Wal - Mart and Target. They have backpacks from 10 dollars to 20 dollars which is pretty cheap compared to mall standards and high class backpacks. All you really need is a simple backpack.
The cheapest backpacks for hiking on that site are made by Teton.This is a good make in the hiking world.They supply rucksacks and holdalls as well as backpacks.
School backpacks can be purchased at any store that sells school supplies. Walgreen's, Wal-Mart, and Target are all stores that carry backpacks. Stores that specialize in items for children such as Toys R Us and Kids R Us also will carry backpacks.
The store I think is the cheapest is BigLots. They have a ton of stuff or even dollar stores have a lot of school supplies.
Jansport sells approx. 4,000,000 backpacks per year
Walmart sells tons of different backpacks
The North Face sells backpacks that are usually made for people who climb or sports on mountains consequently their backpacks usually are military-style.
There are plenty of budget brands. Jansport sells rolling backpacks for very little.
Amazon is a great place to look for laptop backpacks. This is where you will find the largest variety. Best Buy also sells laptop backpacks; both online and at their retail stores.
My buddy risky...
dakine
You can get the cheapest stair stepper at pricegrabber.
Some schools do not allow backpacks because they worry that weapons can be hid in them. Some school allow it if the backpacks are clear.