I would think it was a great rarity for a cartographer to receive a royalty. Even cover artists usually don't get that, although I understand that it is not uncommon to put a stipulation in the contract that the art can only be used for a specific edition of the book.

Look out for the phrase "work for hire" in contracts. That phrase means that copyright belongs wholly to the client, and you have no rights to your work. They can continue using it forever without compensating you at all, and you cannot profit from it in any other way (such as publishing an art book of your own). I won't say never to sign a work for hire contract, but if you do make sure you're handsomely compensated for selling your rights.