Cheap insulation. Is there such a thing?

Reflectix-
Bubble insulation is ideal for crawl spaces, radiant floor heating, and basic floor heat retention applications as well as for insulating pole barns, post frame, and metal and steel buildings.

This is what I used on my Metal Pole Barn. You don't want to use regular insulation with metal walls it will cause the walls to rust.

Here is what it looks like , this is not my Garage just a picture off there site.