Cheaper source for Styrofoam - Maybe
If you are looking for Stryofoam for a raft-type system and you have a drywall supplier in your area, check them out. I picked up several 1.75" pieces, some as large as 8'x32" that were damaged - for nothing. Even buying them new, they are at least 20 percent lower than HD or Lowe's sell them for.
I'm just doing an experiment now, about 18 plants in a 20x30" box using a simple DWC system but if it works, I'll have at least 96 sf. of space for lettuce/chard this coming winter.