I don't really see a point in counting the average sales per album.
anyways , when it comes to the "pecking order" in Avex Kuu is definitely above Ai. I might have more consistency in her single sales , but she hasn't reached massive sales like some succesful Kuu singles like Koi No Tsubomi & Yume no Yuta /Futari De. regardless of the average album sales , Ai doesn't come close to the success of Kuu's both BEST albums (which account for like 70% of Kuu's total album sales

).
imo , Ayu's been so much longer in the business there's no point comparing her sales to Kuu's or Ai's.
also , whats the source for these sales numbers? they seem a bit low for especially Kuu (secret sales are in the 500 000 , the BEST's both sold over 1,5 million each , Black Cherry is at least 900 000. I think even if you leave out the first BEST and count only the original albums Kuu's sales should be close to 3 million ...)