Hi, I have very limited experience with casting but I thought the only good option was the silicone and casting resin as you are using. I myslef have googled quite a bit trying to find cheaper options but came up trumps. The mold making silicone used is designed for the job so it gives the best results by far. There are various different casting resins however, but I think the silicone is all the same. I could be wrong but I am pretty sure even the pros all use the silicone.
As for making molds of really big items people normaly "jacket" the mold, otherwise known as matrix molding. Its a technique where you only use a thin layer of the silicone around the piece (around 1cm thick or so) and then hold that layer together with something like a fiberglass jacket. This is done purely to minimise the amount of silicone used, since as you said it is expensive.
Google it, there are loads of tutorials online xD. Punished props is a good place to start.