I started doing card art of people's wyrms, in colored pencil (see my posts for examples) and I charge $25 for them. For a good-sized piece of art that's not too bad, I think. (I think my originals were about 6x8 or so...).
I think digital needs to be less, as the client does not get a physical piece, and the process is (usually) faster. But, I don't do digital art, so i can't speak to that very well.
Charge what you think will get you more business as you start out.
