Here's an article on a survey done of 1007 self-published writers. Fewer than ten percent of them earned a living-- but if I may say so, that's actually a remarkably high number. Five years ago, practically no one was earning a living from self-publishing. And it's not as if going through traditional or small presses is a guaranteed way to earn a living, either (though I do suspect the figure there is higher than ten percent). It's worth noting, though, that half the respondents earned less than $500, so there's a big disparity there.
In any event, having an agent seems to help boost earnings, as does investing in editing, cover design, and book trailers.