Publishing cost by Apple.

Apple will publish your eBook for free through iTunes Producer. if your eBook doesn't have a purchase price, then it is completely free. But, if your book does have a purchase price(you can set the price in iTunes Connect), Apple will publish the eBook for free on Apple Books but take 30% of your affiliate earnings and total earnings combined. Hope this answers your question. Thank you

There is no charge to publish your book. If you sell a book, Apple takes 30% I think.

(Apple's Book store only sells ebooks, they do not do paper books at all).