Using the cheapest stuff possible that is able to re-use as a mold, and using the cheapest stuff possible to cast. Plus the stuff needed for releasing and sealing. How much would it cost to make a helmet out of the stuff?
I don't think plaster is good enough to capture details, even less when it comes to molding things with undercuts like the MC helmet. If you can do multi-part molding, maybe but since plaster hardens like a rock when it dries, glove molding is out of the question. It's more appropriate for making mother molds where rigidity is a desired quality.
Might I suggest latex mold builder? It's not as expensive as silicone rubber ($67 per gallon vs $160 per gallon of Rebound 25) and captures details very well. On the downside, it's a time-consuming process and I emphasize here TIME-CONSUMING, to put sufficient number of layers of latex for glove molding. In addition, it's quite delicate and not as robust as silicone rubber, thus giving you much fewer mold casts.