it's hard to estimate ayu's income, but i'm pretty sure what she gets from her music sales is not that important compared to what I'd image she makes off her merchandise & tours.
I'd estimate that one commercial endorsement from Ayu is worth at least 1 million US$. the pay for doing commercials in Japan is so good that plenty of western artists/celebs do them too. for many j-celebs doing commercials is their main source of income.
There's an article about Yu Yamada on Japan Times that estimates she makes 300k-500k US$ per campaign, and while she is very popular (not her music tho lol) I think Ayu would still be in a completely different league than Yu.
Also I'd imagine that Ayu's been given shares to avex at some point when she was making 40% of Avex's income. it's all just speculation, but it's very common in the business world to award the key people in your business some shares to make them stay & commit to your company.
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