I actually also read that Rumiko Takahashi is the richest person in Japan. That info is pretty old but it did not specify whether or not it was based only on manga-ka or not.

With Ayumi, she keeps a large entourage, so that comes out of her pocket unless Avex is the one that takes care of those charges. She's probably still one of the top earners in her business but I doubt she earns as much as she did in 2001.