That 40% gets quoted alot, but 1) as I recall it was revenue, not profit, and 2) that hasn't been true since about 2001. It's one of those things fans throw around to show newbies to jpop how profitable Ayu is when that's definitely not the case anymore. In 2009, ayu was outsold by arashi, exile, and several others, which automatically mathematically means she couldn't account for 40% of what avex makes.
The (all english) avex investor's guide provides some data & facts relevant to Ayu, showing her stance in the company, although it doesn't give any hard numbers vis-a-vis sales numbers, units shipped, total yen earnings, etc... should give you an idea though.
http://www.avex.co.jp/html/upload_fi...80104_P01_.pdf