Cheap insulation. Is there such a thing?

So is the structure metal? Can you put a basic frame up? Sheetrock will help a great deal.

Not sure if foam board insulation is cheaper than fiberglass rolls, probably almost the same for the same area. You'd probably only need 1/4" or a lower R rating since the temp swings in your area are not that great. Your cost obviously would be in volume.

My garage is wood framed and unheated but it is insulated and sheet rocked which helps regulate the temp pretty well. Sure it gets very cold or very hot but the temp inside does not change as fast as it does outside which mitigates condensation somewhat. I don't remember how much it cost to do my 20x22 garage but it was not terribly expensive, probably a couple hundred bucks for the sheet rock and insulation. I did the job myself.

My ceiling is uninsulated though - obviously I lose a lot of heat there when its cold and it can get really hot in there during the summer. Poor construction in my house made it tough to do right so I went without. If I was to re-do it, I would insulate the ceiling.