It completely depends on the artist and styles you’re looking for.
I have commissioned several artists through Instagram all at varying prices. I’ve also been in touch with others regarding their fees for future works.
If you’re looking at a more realistic style, then the price tends to be higher. And if the artist is a professional and their work is of very high standard, then (also depending on amount of body showing/characters/details) you’re looking at anywhere really from $60-$300 etc. It’s such a huge variation.
Also you will find that some well known/professional artists charge a percentage on top for allowing it to be used commercially (in episode in other words) this tends to be 20-30% but again can be more.
Searching Instagram, you are likely to find a higher fee, as those artists are professionals who usually make art as a job, or sideline and have lots of experience, where as artists on here mostly do it as a hobby, or are still learning. That’s not to say there isn’t good artists on here of course.
My suggestion is to get together your information that you have for your cover design you want, and then contact the artist that you’d like to commission and make a query. Ask them politely what they would charge for it, and see if that is a budget you can manage.
Again, not saying you would do this, but Do NOT attempt to barter the cost down, or say it’s too high would you take $X instead/ payment in smaller amounts etc. This won’t work with professionals and is just rude.