The last time (before the pandemic even) I looked at glass and acrylic/polycarbonate sheets for a project, the glass was cheaper for a similar size. Maybe there are other plastics out there that are cheaper, but I'm not sure that they are suited for similar use. If they are suited to this type of use and are cheaper, then someone can make a good fortune if with the right marketing.
Oh, so not the material nominal pound for pound cost?
What do you mean? From a consumer perspective dealing with sheets, one requires dimensions for a project, not weight. Whether measured by weight or by dimensions, the glass was cheaper.