Cheap insulation. Is there such a thing?

You didn't say what you have for interior walls. I'm imagining just a corrugated steel panel between inside and outside. If that's the case, anything you add to the inside to create a dead air space between the walls will be a great help. Keep it white though. Black poly will soak up all the light.
Next time you're at a convenience store, look at their temporary Coca-Cola and Budwiser displays. That material is a corrugated styrene, about a 1/4 inch thick and contains its own dead air space. It is a great insulator. Light weight enough to install with most any adhesive.
It is quite flammable though, doesn't pass any fire code.