FWIW, Mcmaster Carr lists regular polycarbonate at about $60 for a 1/4" x 24" x 48" pc and "abrasion RESISTANT" polycarbonate in 1/4" x 24" x 48" at about $120.

Why don't folks use plexiglass (acrylic) for the side and rear windows as it is cheaper (most hardware stores carry it) and more abrasion resistant than lexan (polycarbonate). Plexiglass will break unlike lexan, but for rear quarters and rear windows why would anyone care? Just something I always wondered about.

I have lexan in one experimental airplane and plexiglass in all the rest and the plain lexan definitely scratches easier than the plexiglass. Most general aviation windshields and windows are in fact acrylic as opposed to lexan believe it or not.

That's a very good point, and I think I'll simply go buy some Plexiglas for those quarter windows. After all, I've sat behind Plexiglass for over 3,000 hours in the air and it never bothered me...