It doesn't work like that.
Anything limited to the one story you are writing, is basically free. Only things with persistence beyond the story need to be paid with Prime. So you can just invent a village and write about it - but if you want to put it on the map, and revisit it in a later story, that is where you pay Prime.
Prime scales with depth and detail, not with quantity. A settlement on the map has the same Prime cost irrespective of size. But the more attributes you add to it, the more Prime you spend. The more "bland" something is, the cheaper. The more depth, details and story it has behind it, the more expensive.