Cheap insulation. Is there such a thing?

In the Georgia heat, I think the cheapest and most effective temp control isn't insulation and a/c. I would spray the outer surface of the metal roof with highly reflective silver or white paint and install a high CFM exhaust fan either in the roof or as up high in one of the gables as possible.

If you do go with insulation, insulate the roof 1st as most of the solar gain will be through that roof. But paint it 1st.

Your issue in summertime is the sun heats that roof and the roof heats the air in the shop. Limit that solar gain and get that hot air out of the shop and you'll see a nice improvement for pennies on the dollar vs insulation and a/c.