Cheap insulation. Is there such a thing?

Here's where i'd start.

Energy star reflective coating
http://www.ppg.com/corporate/ideascapes/metalcoatings/products/coolmetal/pages/default.aspx

Here's some fiberglass insulation that is designed for metal buildings. If you do it a little at a time, do the roof 1st - best bang for the buck. But you should install an exhaust fan high up in conjunction with it or you wont see the full benefit.

http://www.insulation.org/metalbuilding/pages/install/install.cfm