I sort of doubt that it was polyethylene since this is more difficult to foam. Maybe polypropylene...anyway, in that family, no doubt. I lost about half the thickness over 6 years, so I am really looking for a UL replacemnt. It is difficult to compare, based on a generic name. Clearly, by weight, that was not what I have been using.
I think the 3/8" (avg) value for the convolutions would weigh more. Check my numbers. (3/4 / 2)=3/8" in an even distribution. (Given 2 layes meshing with each other.)
Reducing the weight by drilling 1/2" holes through the PEAKS will do more for lightening weight than through the valleys. You really only loose a little R value, maybe R1/2 or R3/4 for about a 3 ounce savings. Fine for any 3 season pad. I don't use it when temps get much below 30F except to suppliment the NeoAir. In warm weather, it is fine, except in lean-tos...getting old, I guess.