Just as the price changes on the "(P) - Price" when selecting a different variant / size, it should also change on the ATC button. There should be an option to show (P) Price in the button, which dynamically changes when selecting a different variant. It could look like on the screenshot attached. However, I would make a long " - " instead of this vertical line. (But maybe there could be the option to decide between the two and maybe a dot).
I would like to thank you for your suggestion on improving GemPages' User Experience.
Currently, GemPages doesn't allow shop owner to dynamically change ATC button's text. I've well-noted it down and will discuss with the team on when is the appropriate time to develop it.