You are looking at this the wrong way. The more expensive she is, it means the less traffic she is getting, and doesn't feel like any guy of the street has been with her. If she charges everyone the same way, then she might be just seeing one or two clients a day rather than a swarm of clients that have run a train on her, punctuated by $$$ and next in line, where you'll feel sick you dealt with her when you are client #15 that day with a lower price. It's better to spend a bit more on a great experience then feel sick with yourself afterwards.
Anyway...if you are just dancing with her, because of your "morals", then paying that much money to dance with her does not make sense. She is implying full service / sexual, not some innoscent dance. At this point you can tell her you are engaged and loyal to your other girl.