I know that the sim-fire kit involves about 300 dollars worth of internals. Solenoids, switch, tubing, wire harness, chamber, Etc. Control boards tend to be about 100 dollars. Motorcycle battery about $35. $200 ought to buy you a used oxygen bottle, two regulators, and about 35 feet of hose with flash arrestors. Maybe $15 for the ridiculous fitting to go from propane bottle to 1/4" hose fitting. BTW, the longer the hoses the better, it prevents "starve-out" and the dying mule sound most of them make. This can also be done by building an accumulator cylinder between the tanks and the gun. $400 for a 1919 kit plus replacing the barrel with a DOM tubing replica that has the combustion chamber on the end inside the sideplates. Maybe around a thousand or so? I had a 1917 gas gun that was awesome. Super loud and reliable. Sold it with the tripod for $1500. Ooooooooooooffffff. That still hurts.
Run propane at 5 psi, Oxy at 12 psi up to 25 psi depending on size of chamber, how many beers you had the night before, Etc. Frazer42