subscribe to gamedeveloper magazine - they do yearly survey of all agmedev related jobs, art included. Also maybe they have that for free... dunno, u can check on google for their surveys.
hope this works: http://gamedeveloper.texterity.com/gamedeveloper/fall2011cg#pg25 it really depends on the level of the artist, the location, the studio, and other factors, but i think you'll get a decent gist from the survey :)