About 9 years ago, I stumbled on a huge re-roofing job at the local high school. They were removing the old 1-1/2" thick, 4'x8' sheets of foam. After finding the jobsite Superintendent, I found out they were giving it away. I rented a trailer and brought home over 150 sheets that I used to insulate my workshop. Another campus had torn out 6" thick John Mansville, foil-faced fiberglass insulation. Using a combination of both, the West and North ceilings and walls now have an R-50+ rating. These days, one (free) thick sheet is equal to two 3/4" thick foam sheets sold at the Home Centers for $20+ dollars, and would have cost me Thousands to insulate my workshop, well house, and two-car garage!
If the foam is in sheets, it's worth good money. Run it on Craigs list or a big ad in a large Metroplex newspaper. It seems I saw on either "Fine Homebuilding" or "Mother Earth" website where a junior editor bought used foam for his workshop from a secondhand supplier in the Northeast. Even with his shipping charges, he was happy as a lark!
One word of caution: The reason I got mine for free was because at first they tried to stomp it into pieces that quickly filled up those huge and costly debris dumpsters. The second reason was that when they realized they had to start stacking the fiberglass-mat coated foam sheets and keep them out of the debris haulers, the wind was their enemy.... Even tarping and staking down the stacks of foam sheets didn't help much, so sheets and pieces began to scoot and fly across the empty student parking lots... They were happy to get them, and the 8" long steel screws with 4" flat washers, out of their way and off their responsibility....
Be sure to have a good and dry place to store and stack your foam sheets while waiting to sell them.
Bill