I have some left over 1.5" thick XPS insulation (aka "blue board") from a recent project.
1 full sheet 4'x8' $25
1 half sheet ~4'x4' $15
"new" price is about $45 for a full sheet I believe.
I also have a number of "cuts" ranging in random sizes from very small 6"x6" to larger pieces about 12"x48". These pieces are free.
$30 if you take it all.
Text Mike at 206.618.0858. I'm in Mazama.