I prefer sculpting with milliput, but that tends to be expensive on larger pieces too.
I generally build up as much as I can with cheap materials - card, bodyfiller, whatever - then add the more expensive milliput as an outer layer. Still get through a LOT of the stuff though.
Other alternatives, if you are molding and casting, are chavant and those types of clay. The good thing about using that stuff is, once you are molded up you can scrunch it together again and reuse it.