At my local Ace Hardware store today I bought a window insulation kit package (plastic plus double-sided tape). Advertised as a 6-window kit it's 108 sq ft, 62" x 252", and just cost about $13 after taxes --- not bad. This is actually branded as an Ace hardware product.
My GG Polycro is 40" x 96"; GG lists this as weighing 43g, and I weighed mine at 47g. That's about 82 square inches per gram, or 0.012 grams per square inch.
The Ace Hardware plastic is about 70% of that unit weight at 0.008 grams per square inch, and indeed it feels a bit thinner to me too, though my old GG polycro has been through a lot and is doubtless a bit less flexibile/pliable.
Given the even lighter weight, I'll be happy to try this alternative, but I'm not confident that it will wear as well. If anyone has weight-per-square-inch info on other brands, that might be interesting, i.e., maybe there's are particular brands that are about equally heavy duty as GG polycro. My 62" x 252" piece from Ace weighs in total 132g (4.7 oz) fresh out of the box.