It's plain straight prostitution. Don't understand me as criticizing it, on the contrary... I realize that there is a stigma attached to it but hey, there is a stigma attached to BDSM :-) There's nothing to be ashamed of from a master point of view. I think we have to be mature enough to look at it the way and exploit the turn-on side of it... It is prostitution and it can be fun among adults.

Besides, you might want to emphasize this fact to your slave. You can label and humiliate her this way. The shame that will follow will be part of a richer form of submission, which wil make the whole thing even more interesting.