The logistics of mistressing

So, If Shopgirlove does sell her soul, how much is it worth? And what kind of commitment does Shopgirlove have to make? My understanding is that mistresses are allowed to date, but that the keeper of the kept woman has the right to preempt any and all previously scheduled activities, kind of like being on call. Then there's the question of remuneration. I suppose this depends on the length and nature of the relationship.
Katherine Hepburn and Spencer Tracey had their thing, but what about a less spiritual connection? Is the woman than entitled to more? It's obviously a dead-end relationship, so she's sacrificing a shot at finding a long-term partner...does that mean she deserves a reward for investing in something shady and dubious? Or does that mean that she should have a guarantee for giving her time to someone who will never, in the long term, be there for her when the chips are down. Is there a professional group for kept-women? Can I get some answers here, people?
How ironic is it that this is the subject of my blog on Martin Luther King's holiday. Although, he was not the paradigm of faithfulness, I understand. I wonder how his mistresses fared?