What temperature of sleeping bag do you want. For three seasons, what bilk said. A summer bag would be much easier to make and has a much greater margin of error. Of cource the difference in cost between making a summer bag and winter bag are not all that much (one more layer of insulation). If money is the only concern, then you can get some very quality bags very cheap these days. However, if doing it yourself is the point, then that is a different story. Unfortunately, it still costs money to make your own. The real benifits in cost come after you have done a few prodgects and have better skills, know exactly what to buy and what not to buy, and a plastic bin full of extra fabric.
The cheapest options for sleeping bags are quilts. Two layer of shell material, one or two layers of insulation, thread, possibly some velcro, cord, and maybe a zipper.
Scott