My totally unscientific feeling on the subject of average self-published sales is that the situation is skewed by a considerable number of incredibly unprepared writers who self-publish their unspellchecked first novel, sell a few copies to their families, then drop off the face of the earth. For this group, lifetime sales are probably well under 50. I think that must pull the average dramatically downward. If you counted only serious self-publishers, even for a very relaxed definition of serious, I suspect the numbers would look quite different.
I know a group of writers, all previously or currently published by commercial houses, who are now self-publishing their backlists and some new titles as well. That group of writers is doing quite well on the whole---some making in the mid-five figures annually. There's a
post on my blog about this phenomena by bestselling author Lorraine Bartlett, who told me she's making far more now than she did when all her books were traditionally published. But writers who were or are traditionally published are better writers than your average bear.
The writers I know whose work has all been self-published haven't done well at all and many have been reduced to screaming constantly on Twitter and FB for folks to buy their books. It's not a pretty sight.
I don't read enough self-pub'd books to know if you're right that the unprepared writers are the exception, though I have to say I doubt it. (I've read half a dozen, all by friends, and smart as my friends are, I didn't think any of their work was publishable. And spellcheck has nothing to do with it, btw.) I spent 14 years as an agent, and the last thing I ever want to do is troll through a slush pile again, which is rather what the pool of self-published writing seems to me.
But the closest thing to real stats I've ever seen came for a piece in the
Taleist on the results of a large survey. "According to the survey, 75% of all royalties generated by self-published books went to the top 10% of writers; and half of all self-published writers earned less than $500 a year."