What I use isn't necessarily cheap, but it works fantastic. I make 4'x8' frames using 2x4s spacing them every 16". Then, I put 1/2 inch OSB over top. It gives you a nice solid floor, and something to screw your walls into.
Otherwise, what we did our first year was us pallets to put our props/set items on and then filled in the rest of the space with wood mulch. It worked out great, but we had a bunch of mulch left over after Halloween!