(I found my reputable artist by picking a trade novel cover I loved and reaching out to the artist. I recommend trying this in lieu of a list.)
I believe that most cover artists in the 750 range are reputable.
For an ebook, you can buy a premade cover of reasonable quality for 100 dollars. But for something from scratch, probably 400 will do it.
I pay in the 750 dollar range for novels, and that includes ebook and wrap-around (print.) For the linked novellas that I plan to release in e-book only, I plan to find something much more affordable.
A good cover is important, but the pre-made covers, while not 'the best,' certainly qualify as good covers. You scan through thousands and find one that works for your purposes.