Publishing is a costly business. You need editors (multiple), graphic designers, production, marketing, etc. On his blog, Assaph has done a critical analysis on how to efficiently publish a book — without breaking the bank. It’s an excellent summary on how to get into the pro league while staying competitive. If you’re considering self- or hybrid-publishing, we highly recommend you read this post:

https://egretia.com/2018/10/30/the-costs-of-self-publishing-and-how-to-reduce-them/