Cheap insulation. Is there such a thing?

I am in middle Georgia so it does not get too terribly cold for too long. But it does get pretty hot in the summer. My shop is made from one of those metal awning kits and was enclosed. 24x28 with 12 foot peak and about 9.5 foot sides. Two 10x10 roll up doors and one 4 foot personnel door. Is there a cheap insulation I can get and do a little at a time that will help keep the temp stable? Maybe help if I ever get an A/C unit or some heat out there?