It depends a) what you're looking for and b) the quality of the artist. As a professional designer, I charge around £300 for a book cover ($400) and in the region of £150 - £200 for an ebook cover. At times, when I do low-budget work, this can be lower. But then I don't provide pre-made covers and others do (though they regularly look pretty bad) and these come across as much cheaper. When commissioning illustrators and artists for clients, the prices vary wildly based on what they're doing, a simple illustration can be as little as £40 while a more detailed painting might cost just as much as the cover. The issue with cost is that very few professional and skilled designers would drop to these prices, on account of it really underselling their work, so many people get stuck either doing it themselves or hiring under-qualified designers/illustrators for a noticeable drop in quality.
In a publishing setting, the publisher would do all of this. They would commission designers, whether in-house or external, and put through briefs on art. As an author you wouldn't have much involvement other than to look at it at the end. You might be allowed to confirm a cover, but sometimes even this doesn't take place.