Different items in the Crown Store have different costing procedures, factoring in such variables as intended audience size, balancing content play to convenience, in-game utility, window of availability, etc.
Dungeon game packs have a smaller audience size than overland game packs, so the dungeons are cost more crowns.
In-game motifs have more in-game utility and requests than seasonal motifs, so the in-game motifs cost more crowns.
Motifs have a smaller audience then game packs, so the motifs cost more crowns.
I know, some people would love to think all it takes is a dart board and some evil laughs. But there's an appropriate methodology to it.