Cheap backpacks can be made, but I really do not recommend it. Unless you're really skilled at sewing, it's unlikely to come out like you need. Acting Classes contains more about when to recognize this activity. Also, even if you are a professional, it will probably take less of your time to pick up a supplementary day at work and buy a package using the profits.
On-the other hand, if you've an old backpack shape, you can make a cheap backpack with it. Eliminate the old pack, and tie a plain nylon duffel bag to it firmly, with the zipper facing out. Small bungee wires works for attaching it also. I did this using an old aluminum frame that still had a belt and straps, and for the price of a duffel bag, I had an external-frame backpack that not merely used a lot, but considered only two pounds.
Getting Cheap Bags
There are several methods to buy cheap backpacks. The most obvious would be to just watch for an excellent purchase. A faster way is always to shop for a used package on the web. Take to Ebay.com, or visit a backpacking or other outside community which allows visitors to sell their gear, like whiteblaze.net. The boards are nice, since you can quickly ask questions regarding the pack.
You can also get used backpacks cheap at rummage sales. Watch the ads for any mention of outdoor equipment, and call to see if they have a backpack you might want. You might be in a position to get it prior to the sale begins. Usually, start shopping early, and discuss somewhat. I recently sold an attractive Kelty frame-pack for $15 in a garage sale.
Cd stores often have low priced bags. More often they have time packs, but you never know. I've seen large old frame packages that were rough at thrift stores, and it happened to me that for utilize the frame having a duffel bag, as described above, and a few dollars I could just throw the package.
Still another method to keep the cost down is to go light. The lightweight backpacks, unlike other lightweight gear, are often cheaper than the big packs. You can conserve money too, if you have been thinking about lightening the weight. Ultra-light bags usually are under two pounds and frame-less. You'll probably use a sleeping pad like a "frame" of sorts. The good thing is that they're usually on-sale for under $100.
Go-Lite has many packs that are near $100, and often on sale for under that. The list price on the Granite Gear Virga Ultra Light Packer might be under $100, and I have observed it on sale for under $80. It also weighs only 21 ounces, a bonus for me, as I just like the concept of light AND inexpensive backpacks.