If you're going through a publisher, their marketing department does that.
If you're self publishing, you find an artist who will do the cover for you. If they're professional they'll charge you industry rates ---
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The most common range seems to be $250-500. Print cover versions can be additional $50-150 (so if ebook cover costs $299, print+ebook would be $349 or $449). Now, $600 for a book cover is expensive.
Book Cover Design Prices in 2019 - Rocking Book Covers
https://www.rockingbookcovers.com › book-cover-design › book-cover-des...
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If they're a friend who does art, they will charge you less.