title_body: What was the average income from ebooks for independent authors in 2013?
upvoted_answer: Statista (The Statistics Portal) looks like it has some relevant ebooks revenue information (though not exactly what you are looking for). Unfortunately, you are forced to sign up for a premium account to access most of the reports. A sample report that they actually show you some of the information for is here (Global e-book revenue from 2009 to 2016*, by region (in million U.S. dollars), obviously some is projection).
  
  I did manage to find an informal survey done by Marie Force about independent publishers here (covers total unit sales broken down between 2010, 2011, 2012 and some of 2013) . Here is an excerpt:
  
  
    The conversation on the loop led to me post an informal survey to get
    an idea of how our members and their self-publishing friends are
    really doing. I didn't ask people to give dollar amounts, but rather
    just their number of sales in 2010, 2011, 2012 and so far in 2013. I
    asked them to list their most successful book in 2012 and how many
    copies it sold. At the outset, I want to say that this survey was
    informal. There is nothing scientific about it, but it does provide an
    interesting snapshot of how self-published authors are really doing.
    
    To give context to the numbers, I wanted to also provide a guide to
    what the most popular price points translate to in actual dollars.
    Using Amazon's conversion chart, here it is:
    
    0.99 (35 percent royalty): $0.35 per sale
    1.99 (35 percent royalty): $0.70 per sale
    2.99 (70 percent royalty): $2.04 per sale
    3.99 (70 percent royalty): $2.74 per sale
    4.99 (70 percent royalty): $3.44 per sale
    
    While I didn't ask authors to list their most common price point
    (which I will add to the survey next time), I thought the breakdown
    above would be enlightening to the authors out there who might be
    waffling about whether they ought to try their hand at
    self-publishing. If you are thinking about whether or not you should
    dip your toe into the revolutionary waters, ask yourself these
    questions: When was the last time I made $3.44 on the sale of one
    book? When was the last time I made even $2.04 on the sale of one
    book?
  
  
  She then goes on to post a bunch of survey results broken down by author. Here is a sample of one of the survey results:
  
  
    Elisabeth Naughton Romantic Suspense and Paranormal Total number of
    self-published titles: 12 Total number of self-published sales in
    2010: 0 Total number of self-published sales in 2011: 0 Total number
    of self-published sales in 2012: 102,179 Total number of
    self-published sales so far in 2013: Jan-March 2013: 370,059 Most
    successful book in 2012 and how many sold: MARKED - 30,673 copies
    
    I've been traditionally published for four years and my income comes
    primarily from my self-published titles, NOT my traditionally
    published titles.