It is cheap, and it's mostly sound insulation and will afford very little thermal insulation. Some, but not really what we need out here.
If you want to insulate it right it'll cost you between $400 and $800. Foam boards, canned foam, and either Reflectix silver bubble pack, or about 5 or 6 of the do it yourself Thermo-mat Isoflex kits found here
Sprinter Cab Window Insulation Sets - for 2001-2008 all models * EuroCampers.com
and here
Window Insulation