Actually, I cornered a professional illustrator at a reception last night. He suggested trying to retain some sort of rights (simply because you never know), but to start with a high number per character--and that it should be per character--like $10,000. Then let them talk you down from that.

(He also said medium-sized outfits are the hardest to work for, because they tend to do these "why don't you give me a number" things instead of having a set price or a price that they themselves decide--which would be the method of large or single-person clients.)