Its the salary of the best GKs, among which he is.
Ter Stegen makes 18m. Oblak 10m. De Gea 15m. Alisson signed his contract a couple of years ago for 8, when he renews he will be above 10. Approximately, according to most sources.
If Dybala can get 15m from a different team then it makes sense that he asks for 15m. I don't think he is worth 15m amd don't think the club should offer it as there are enough ways to replace him, but if that is what he is worth for other teams then let him go for it. It has little to do with Donnarumma.