Foam on the range
The most I ever paid for that foam was like $300 for a 60" x 24" x 80" piece.
You can usually get that stuff on the cheap if you find a company near you. Look up Specialty Insulation Products in your yellow pages an make some calls locally. I found a place near me that makes the stuff, and I talked to the shop supervisor and asked him if I could rummage through his scrap for usable pieces, and he considered it a favor for him! Eventually all I had to do was call him up and ask him what size I needed and he'd save me any scraps that were close to that size.
Places that manufacture that stuff get drums of the liquid chemicals that they mix on site that expands into big forms that they create. Then they'll take those big blocks and cut them to shape for specific orders that they have. Theres ALOT of scrap, and if the pieces are too small they'll just throw it out. Plus you can take a bunch of scrap pieces and cement them together with that 'Great Stuff' expanding foam and make bigger pieces out of it. Seal it up with a few coats of latex paint or gesso, and the resin won't melt it.
To me wood is more expensive by comparison, and takes so much longer to shape.