Prices vary widely between artists and also depend on the type of asset, animation, size, detail, etc. An animated pixel art sprite could range from $50 to hundreds. What you should do is browse some freelancers'
portfolios, find ones whose styles you like, and send them sample images of what sort of thing you are looking for. Ask for a quote and make sure to be as specific as you can about what you will be wanting, what kind of relationship you are looking for (one-off, steady), and what your budget is.
How the artist decides to handle iteration also varies. Some artists may not charge you until you are happy with what they've done and others may charge you by the hour regardless of whether it was what you wanted. Just be sure to communicate well and be clear about expectations.