I'm publishing a book. Where could I find an illustrator for the front cover? How much is the cost? ?

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.

You can watch this video: https://www.youtube.com/watch?v=ZkoltFuljlE
In 11:05, it talks about cover designers

Find books similar to yours with covers you like and find out who the artist is. (Usually the author will thank them in the acknowledgements or mention them on their website.

Fiverr has a bit of a "bargain basement" reputation - you can find very cheap artists there, but a lot of them aren't very good, and aren't above stealing other artists' work.

The price depends on what you want and what sort of reputation the artist has. You can get a ready-made cover for a few tens of dollars. (The artist makes lots of genre-specific covers in advance from stock images, then you pick one and they put your name and title on it.) At the other end, a custom cover, where the artist paints a picture solely for you, or hires models, props and costumes to create a photograph just for you, could cost a few thousand dollars. If you're just starting as an author, it's probably not worth spending more than a few hundred dollars for a cover, since you might struggle to sell enough copies to show a profit otherwise.

(To clarify a bit, using stock images isn't "stealing other artists' work." Stock images are meant to be used in many different people's projects, and the people who create them know they're going to be used like that.)

You can easily find artists online, either by just searching for "freelance artist" or checking websites such as Fiverr.