Cheap insulation. Is there such a thing?

Contact the people in your area that build and sell metal building supplies. There are several In my area. They usually insulate as the building is assembled. Some use plastic backed, some use foil backed and some use foil/bubble/foil. All of it is pretty low in R-value, maybe R6. Most fiberglass for 2x4 walls is R11-R13, and would have to be enclosed.