It varies; a lot depends on what you are looking for, and how detailed/high quality you want the work to be. First rate art, nicely inked, colored and shaded with backgrounds can cost $150 per pose; simple cell shading, no backgrounds, knocks that way down; just pencils, even cheaper. You CAN, if you are lucky, find an artist to do some decent colored drawings for $15-25 per pose.
"Raz", who advertises his comic stories on this forum, works at about that range, for colored art.