Labrigger has a terrific post about replacing the Kwik-Cast platinum-cure silicone elastomer ($106 for 5 mL) with Body Double Fast Set at ($30 for 400 ml). Depending on your dispensary needs, you might want to purchase a 400mL preloaded dispenser cartridge directly from smooth-on which goes with a dispenser gun for $70. Or buy a starter kit to get it all together!
We first came upon these sorts of materials when discussing the use of expanding foam for fabrication with some members of the DIY cosplay community. Focused on cheap, quick and simple, the industry that has grown around this hobby is absolutely terrific.