I think you'll find that rates for cartography commissions will vary considerably by artist, skill level and the nature of the project. While I don't consider myself a professional I have done several commissions over the years, some of which have been for independent RPGs, and the lowest rate I've ever charged for a commission was $100 USD (and that's only because I was friend's with the client). What I do is start with a base hourly rate, calculate it by the amount of hours I think a project will take, and then modify this by other factors such as how soon the commission needs to be completed (I charge extra for commissions that need to be completed quickly and less for those with flexible deadlines), and whether or not the client is seeking to license the piece or wants the copyright to it.

Cheers,
-Arsheesh