In reality, the cost to self-publish a book varies. In my last 10 years as a self-publishing coach I have seen many unique approaches. Although it is appreciably less expensive to publish a book yourself than with traditional publishing, the self-publishing budgets is affected by the following factors:
Book physical requirements like color/black & white, # of pages …
How much technical work author/publisher will do themselves.
Cost to hire services like editing, proofreading, book cover design.
Flow & timing of book project in wrong chronological order is costly.
Adding on other publishing venues such as Kindle, and Audible, etc.
A general range is from “free” (hours and hours of time); to $15,000. The lower end includes massive sweat equity. Notably, the upper end includes a Kindle version, paperback version, and audio book, as well. In addition, the upper end includes full service ghost publishing and/or ghost writing.
For more details, read the articles: Self-Publishing Frequently Asked Questions and Real Self-Publish Costs of Self-Publishing a Book