It costs zero dollars to upload a Word document and a JPEG to Amazon KDP and hit 'Publish'. That is the most dangerous fact in modern publishing.
Because the upload is free, authors often assume the process should be free. But creating a book that readers actually want to buy requires investment.
Where the money goes
If you want a book that looks and reads like it belongs on a bookstore shelf, you are going to spend money in three main areas:
Editing
This is usually the largest expense. Developmental editing, line editing, and copyediting are distinct skills that take a lot of time.
Design
Cover design and interior formatting. A good cover designer is worth their weight in gold.
Marketing
Advertising (Amazon Ads, Facebook Ads), ARC services, and promotional newsletters (like BookBub or BargainBooksy).
Where to save and where to splurge
If your budget is tight, do not skimp on the cover or the copyedit.
Readers will forgive a slightly plain interior layout. They will not forgive a typo in the first paragraph, and they will never even see the first paragraph if the cover looks amateurish.