3/4" 4x8 sheet for $50? Where? That's a crazy good price... I paid about $60 for 1/4" a few years ago. I have some projects going right now where 3/4" Gatorfoam would solve a lot of problems, but I don't have a local source any more, and shipping is too costly for 4x8 sheets. I need to cozy up to a commercial signmaker, I guess.
Neat stuff. Very stiff, very flat, amazingly light. Won't hold fasteners very well, as would be expected with a foam, and any mechanical fasteners going through it will need to be backed with washers. I use polyurethane ("Gorilla") glue to secure it.
I can't imagine building module frames out of it... but I guess it's possible provided you fasten the corners mechanically to non-foam gussets.