If you go to a styrofoam manufacturer and talk to someone in charge, you can occasionally get lucky enough to get free stuff... They stack their foam on huge pallets and if the top few sheets get damaged for whatever reason they'll usually recycle the whole pallet. If you ask nicely they might give you a couple damaged pallets worth. One year we managed to score a whole Uhaul's worth of varying sized sheets (1x4x8 was the largest I believe and we got a few of those) for a Halloween maze we built... had enough foam to build a full castle facade for my house, sculpt four different statues, a row of crypts, tombstones for the graveyard plus the entrance to the maze and had a a stack of foam taller than me left over... Was able to build full scale Ion canons and a STAP and battle droid for an episode 1 promotion at the local TRU (this was a few years ago, lol) with the leftovers and STILL had foam left...
that was a fun year...