That's a very open-ended question. There are loads of artistic styles and dependent on the amount of detail, cost can vary greatly. I'd recommend getting some more definite details over to one of the artists in the art portfolio forum, or post in the help wanted forum. But really first and foremost you need to provide more specific details of what you need.
For the record - and not that this will help - I chose my artist for my current game based on my need for chunky cartoon-style graphics. Also factored in licencing of copyright-free music and a small amount of voiceover work and I think it cost me somewhere in the region of $5,000-$6,000 (I could tell you to the cent, if I could be bothered to sit and add it all up). My budget was $5k so it didn't work out too bad. Can't divulge exact game details as it hasn't been released yet (but soon).