I tried to do enough research on airsoft's and the gas to use in them to make a good decision. I bought my TM MEU knowing that 134A was the recommended gas and that propane was not recommended. I decided to go with propane due to the lower cost and availability. I can't go to a local store to find 134. Coleman fuel is everywhere.
The costs for 134A seem to vary wildly. From .023 to .052/ml. (without shipping costs) Propane is $.006/ml when bought at Walmart. So, after 10 canisters of Coleman fuel through my MEU (with no problems so far), I figure I've saved enough money to buy a metal slide.
I had a Marvel Unit 2 for awhile and sold it and bought a CZ and the Kadet kit when I was shooting my CZ. The Kadet was more accurate and a very well designed unit but I eventually decided I prefer the 1911 platform. I decided to try airsoft as opposed to buying another 22 conversion. The MEU was cheaper by half than a 22 conversion and, when you factor in the cost/availability of 22 ammo, the airsoft is a clear winner in the "bang for the buck" department. I figure, in all of the airsoft shooting I've done, the savings by not buying 22 ammo has paid for the MEU. When you figure in the costs of driving to a range and the ability to shoot airsoft inside where it's warm in the winter (an important factor in Wyoming), the airsoft really comes out ahead.
Bottom line: I figure I've saved enough money shooting propane in an airsoft that if it blew up tomorrow, I could buy another and still be money ahead. If the slide ever cracks on my MEU, I'll go to a metal kit and, in the long run, will still come out ahead dollar wise and will practice much more since my practice range is right here at home.
WyoBob