The average could be as high as 250 if one includes the whales and krakens (whose baseball games will haunt my nightmares from now on). Keeping in mind the long tail that includes most self pubbers, I doubt the average release would sell that many if we exclude the high-selling outliers.
I don't think measuring using our own experiences is necessarily going to be valid. Keep in mind that there are a lot of self pubbers who have no clue what they're doing. In the beginning, that was me. I didn't know how to promote, and, after an initial flurry of 28 sales from people I knew, I descended to 2 to 6 sales per month--and this was back when visibility was a lot easier. At that rate, it would have take me over 63 months (5.25 years) to hit 250 copies on that first book. Many people would get discouraged and give up before then. Also, a book selling that slowly would become less and less visible, so the likelihood is the actual point at which the book hit 250 would be much later than that.
I was estimating the other day and realized I'd sold somewhat more than 10,000 copies of all my ebooks together (depending on how one counts full-read equivalents). Not counting the shorts, which typically don't sell much, I have 11 full-length books, which would be an average of about 909 copies per book. Of course, the sales aren't distributed evenly. Series starters sell more, and one book in particular accounts for a disproportionately high number, so at least some of those books individually are probably below 250. On the other hand, the figures don't include audio and paper, so they're actually a little lower than the reality. However, the reason I ended up doing better than my earliest experiences would have indicated is that I figured out what I was doing, at least to some extent.
I suspect that there are a lot of people who don't. If they don't, they probably don't find WS or the other place, so they aren't posting here or there. I used to see a lot more people on the KDP forum and other places who didn't know what they were doing--to a frighteningly large degree. I vividly remember one author who was wondering why his poetry collection wasn't selling. If you're tempted to say, "Because it was a poetry collection," that's just the tip of the iceberg. It was, if I recall correctly, $6.99 for fifteen poems, and it was a collection of Christmas haikus. How's that for writing to market? Just the thing for sitting next to the fireplace and waiting for your peppermint chop suey to finish cooking. Then there was the cover, which was shaped like a post card.
That's only one example. I saw dozens like that. Most lacked appropriate editing, formatting, and cover design. Most were too short for the price point. Most had odd blurbs. A few were barely recognizable as English. Do we think books like that sell 250 copies? I doubt it. And even a much better book with no advertising would probably still sink to the bottom before it reached that point.
There's no way to know exactly how many self published books are doomed because the authors either don't know how to produce a commercially viable book and/or don't know how to market it. But I would guess it's high enough to make me question 250 as an average.
