I made one - total cost was under $10. A 2 cell battery holder, some wire and a couple of aligator clips, a steel rod from Home Depot, a cork to seal the chamber and a small piece of rubber hose to insulate the rod from the muzzle.
I only use it for cleaning the bores on cruddy mil-surps or other used and abused guns as I use straight household cleaning ammonia. Guns that you can spend forever trying to get the copper out of with a brush get cleaned quickly with the homemade unit.
Straight cleaning ammonia is very high in PH (base) so I don't recommended it for frequent use.