Spot on, GP. Although if I didn't already have a relationship with you, I doubt I'd work without a percentage up front. Not that I do any freelancing at all at this point.

Anyway, there's this convention that you never tell someone how much money you make. A lot of folk believe that you keep it a secret in order to prevent others from undercutting your price and to reduce jealousy. The reality is that it serves to tilt the negotiation in favor of the client/employer. The artists don't know how much to charge, but the clients surely do know how much the art is worth. The only thing they don't know is just how low the artist is willing to go; thus the tactic of letting the artist quote a price. If you want the Guild to serve as a resource for helping cartographers set fair prices and make sure nobody is being taken advantage of, then something as simple as a suggested rate card could do the job. No enforcement necessary; only liberate the information.