[jvehent]

This is accurate. I would add that, in my experience, the value-add of working with a technical editor to shape the book and stay on track is definitely worth the cost of an editor.

And then there are typesetters, copywriters, graphists, reviewers, marketers and distribution platforms that do a lot of the work of making a book successful. Sure, writing is critical, but bringing a high quality book to readers requires a lot of work that you have to do yourself when self-publishing.