Cheap backpacks can be made, but I don’t recommend it. Unless you are extremely skilled at sewing, it isn’t likely to come out like you want. Also, even if you are an expert, it will probably take less of your time to pick up an extra day at work and buy a pack with the earnings.
On the other hand, if you have an old backpack frame, you can make a cheap backpack with it. Remove the old pack, and tie a plain nylon duffel bag to it firmly, with the zipper facing out. Small bungee cords will work for attaching it too. I did this using an old aluminum frame that still had straps and a waist belt, and for the cost of a $15 duffel bag, I had an external-frame backpack that not only held a lot, but weighed just two pounds.
Buying Cheap Backpacks
There are several ways to buy cheap backpacks. The most obvious is to just wait for a good sale. A quicker way is to shop for a used pack online. Try Ebay.com, or go to a backpacking or other outdoor forum that allows people to sell their gear, like whiteblaze.net. The forums are nice, because you can easily ask questions about the pack.
You can also buy used backpacks cheap at rummage sales....