Shoot me an email at Eosin_the_red@cox.net and I will give you a bit more of the lowdown. I am sure that you will find there is a wide range based on your own reputation (chance for additonal work), volume, and the level of skill you are looking for.
A good way to learn first hand is to post and open cal at RPG.net Freelancers forum --- detail your needs and then ask for quotes.
And it's almost as dependant on what you are looking for as who you're talking to. A complex drawing with an action pose, a background and a lot of specific details would cost more than a simple pose without a background. A bust-type head & shoulders view might be very cheap (especially if the artist really enjoys doing character pieces).
While I can't guarantee that I'm average (and I'm a little hesitant to post my lowest rates for all the world to see ), I'd be happy to discuss numbers with you, if you wanted to. I can be reached at calvin@madelf.net
The suggestion to inquire at RPGnet is also excellent advice. There are some very good artists there, who are also good people.