I don't know if this will be useful,
but, from the Dev side, when I use upwork.com to commission artwork, there is usually either an hourly rate set and agreed, with a max amount of hours budgeted to a particular 'character' let's say...
Usually artists, when 'bidding' for a project will provide a free sketch which will take 5-10 minutes to show their vision of the described requirement.
Otherwise they price a few 'revisions' into a final price. So they might say like 200 dollars for this character, which includes: concepts, revisions, sketches, final 4 animations (with 10 frames each)...
Just my 2 cents from working with artists recently.
Viewing post in Art freelance pricing
I guess it would be more risky... that's why I opt for the 'final product' option... I'm just mentioning that many artists DO use the hourly option, and I guess you have to examine their profile, and maybe ask in advance how many hours certain things should take... again I can only comment on what I found as the dev commissioning an artist. I hope some artists respond to your posts with their views.