But those skills can be learned. Negotiation is really an art grounded in science. Artists do it every day, although they take it so much for granted they're often unaware they're doing it. Think about the last time you challenged a phone company customer service representative over a bill. Whether you applied sugar or vinegar, you evaluated the person you were dealing with and chose a course of action.
Some methods work better than others, but it is the desired result that counts.
Some pros are quite good at it. They get those plum assignments with fees bigger than other artists earn in six months. While some certainly have a natural aptitude for bargaining, others have developed those skills the hard way - through practice, practice, and more practice. Then you get feedback about how you're coming across, and the person you're practicing with might come up with an idea you might not have thought of. However, not everyone has someone like Vicki Morgan working for them.