Author Earnings has published a report using data gleaned from Amazon by their Internet spider.
From the article, “What is presented here is but one snapshot of the publishing revolution as it stands today. That revolution isn’t over. These reports can be run so long as books are ranked. Our hope is that the future brings more transparency, not less. Other artistic endeavors have far greater data at hand, and practitioners of those arts and those who aspire to follow in their footsteps are able to make better-informed decisions. The expectations of these artists and athletes are couched in realism to a degree that the writing profession does not currently enjoy.” (Bold text highlight by Ted)
Good information on earnings by authors publishing on Amazon. Whether the results can be similar or not at other ebook retailers remains to be seen.
Still, for authors curious about such matters you may read the full report here.