Yog’s Law is very simple: Money flows toward the author.
The corollary is: The only place a writer signs a check is on the back.
For commercial publishing this is absolutely true, and, I hope, intuitively obvious. Once you’ve moved away from it, you’re out of the realm of commercial publishing. Note: True self-publication is a subset of commercial publication.
(I skip over academic publishing—that’s its own field, and rewards come not in money but in prestige and tenure. You’ll know that you’re in academic publishing when you personally are an academic, you’re writing on an academic subject, and the folks you’re talking with are called the [Something] University Press.)
The next stop is vanity publishing. Here you find the so-called “self-publishing services” along with the true vanities. In this area, the publishers run the gamut from A Very Bad Idea right the way down to An Out-And-Out Scam, with a vast morass of well-intentioned-but-under-capitalized and well-intentioned-but-incompetent in between.
From an author’s point of view, there’s no practical difference between a scammer and an incompetent: Both are time-and-money sinks; neither will get your book into the hands of readers. Worst case: In addition to emptying your bank account you could lose your book for your life plus seventy years.
Yog’s Law will keep you safe from this part of the publishing landscape. Use it as your compass and your guide.
Last is true self-publishing. Yog’s Law is true here, too. Self-publishing is the part of the map where the author is the publisher and hires the editor, hires the cover artist, the typesetter, the proofreader, contracts the printer, buys the ISBN, arranges distribution, promotion, marketing, and carries out every other aspect of publishing.
What you need to recall is that while the author is the publisher, “publisher” and “author” are separate roles. One of the classic mistakes I see with self-published authors is that they don’t put “paying the author” in their business plan as an expense. The money still needs to move from one pocket to another. Those pockets may be in the same pair of pants, but that movement has to be in the business plan, and it must happen. Here, too, Yog’s Law is completely true, and will help the self-publisher run his/her business as a business.
If you can’t afford to put 10-15% of the cover price of every copy sold into a separate savings account, you can’t afford to self-publish.