My answer isn’t necessarily a terribly helpful one. It truly depends one what you mean by average — do you mean the total amount earned by the number of authors (the mean), or do you mean the amount that the average KDP author/publisher earns (the median)?
If you’re talking median — the amount that 50% of self-published authors can expect to earn — it was in 2013 around $5,000 per year or less, according to a report in Digital Book World (Self-Publishing Debate: A Social Scientist Separates Fact from Fiction).
If you mean the mean — the average amount that every self-published author made — it was much higher, because a small percentage of authors earned a lot more than that $5,000/year number.
Self-publishing isn’t a get-rich scheme for anyone. Book publishing — whether you’re doing it by yourself, through a small independent publishing company, or through a Big Five publishing conglomerate — requires an enormous amount of work before, during, and after publication to pay off.
A truism that I’ve heard quoted a lot — though I haven’t seen any documentation — is that over 90% of books sell a hundred copies or fewer. There are lots of reasons for this: many of the books have a narrow audience (a particular family, business, or class) and weren’t necessarily meant for a wide audience; in many cases, the market just isn’t there (the interest is too narrow, the quality is too low, etc.); and in a great many cases, the author and/or publisher didn’t market the book adequately. Publishing a book isn’t just about the writing — there’s a huge amount of work involved in simply making sure that folks know that your book exists, let alone presenting it in such a way that someone wants to buy it.
There are lots of online resources for folks hoping to publish their own books. I highly recommend The Independent Book Publishers Association — it’s got both information and trade resources that I’ve found invaluable over the years. (I’m a member, and serve on BAIPA, the Northern California affiliate organization, but I’m otherwise unaffiliated, for what it’s worth.)
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