According to the graphic artist guild "Hand book of pricing and ethical guidelines edition 15" (page 170) a book cover/jacket would cost a book cover can cost up to $50-$150 as hourly rates, but, or a flat rate somewhere between $100-800 for e-book, original desings. This often includes one insertion, meaing if you have it on your book, you can't use it for copyright, etc.... Normally this would be the cost from publishing houses, and I'm sure you can get other rates form armatures artists. because when you buy an image you do not automatically get the copyright, as you often are buying for personal usage, unless otherwise stated. What you are asking for is one insertion.
With that said, I feel that as an independent writer this might be a bit rough. especially if you are self publishing.
My rates are a bit higher but I would charge a flat rate of $80 for the cover and typography, and give you the image copyright (mainly because I don't want to deal with having to check insertions and I know you may need the image for more than once).
This is my portfolio if you are interested: https://www.cjspiethillustrations.com/
