I'm on staff with two small publishers, have done covers for some private clients, and do my own artwork for my online serials. Please view my portfolio for samples.
Prices vary from $100 - $250, depending on the project. For example, a single cover made with photoshopped stock photography would like be on the lower scale. For full wrap around artwork created digitally, it would be on the higher end. I always give price estimates with my clients when we first agree to work with one another, and always give a detailed invoice for my services.