It looks like SULUK46 does not sell the 1/8" foam anymore. Looking at the Foamorder website, they don't give weights or density. My 1/8" foam from Suluk is lighter than what you have calculated - 40x80x1/8" would be under 4 oz. - if you could get it. But the Foamorder stuff may be a different density and thus a different weight.
By the way - my experience is that a foam groundhseet is great on the snow, not so great for dirt/rocks/pine needles. Especially pine needles - they get stuck in it and you have to spend time pulling them all out. Dirt and little pine comes also tend to stick to it. So I only use mine for snow camping.