Why is this frequently going on sale for 20% on other storefronts, while here on GOG it is only getting put on 10% sales. 20% sales started around November 14 2024 on Steam and hasn't been on sale for less there.

In general the expansions are very expensive compared to other games. To buy the expansions, Expansion Pass 1 + 2, the asking price is twice the base game price. That seems bit excessive.