I'm still not sure if it is worth it as a stand alone venture. :ha, ha: I have gotten consulting clients through my self published efforts, which pays off in dividends.

Generally I make around $50 a month. I'm not laughing all the way to the bank; but it isn't nothing.

I assume most self published [technical] authors are in the same boat financially. There are a few 'lottery winners' out there, such as ngBook, but most probably trickle along.

I'd have to look up dates and run financial reports to figure out how long and how much. I don't spend too much time running the numbers for that aspect of my business.

I published "Life After Flex" which was explicitly targeted to Flex Developers who want to learn "HTML5" applications, primarily focused on AngularJS. That used the "Authority" model; so high price with lots of extras.

I retooled that into the "LearnWith" series which builds the same app in different technologies. It is intended to be "no frills" approach and I sell the books for $6.99 through Gumroad and Amazon.

I'm copy editing the Angular 4 version of it now; and with any luck I'll be working on a React [or maybe Vue?] version before end of year.