There are plans on several of the do-it-yourself sites for machines you can build for yourself for around a hundred dollars. These are stick welders, or resistance welders for spot welding, but they will work pretty well, and they will do good work. I have also done some welding using a truck battery, and if your needs aren't too big, it will work.
I wound the transformer for my resistance welder because that was the only way I could get the precise voltages I wanted to do the welding I needed the machine to do. That transformer weighs a little more than sixty pounds; the machine will handle welds through a quarter inch of metal, bonding a piece of plate precisely to the outside of the thicker part. (In case you are wondering, those are turbine blades I am welding to a shaft....)
The home-built route is a good one if you are a careful and meticulous workman.