Yeah I'm not too familiar with the etiquette either. When I went to the den for a duo the rate came out to $640 for an hour. A pretty good chunk of change for a small amount of time after getting showered, comfortable, etc. While the ladies were ultra-hot the duo wasn't spectacular. At the time I probably wouldn't have tipped as I didn't know better, in retrospect I still wouldn't have tipped even knowing better. In that case I would have only tipped if the experience was very good as the rate was high.
But I also wouldn't want to be one to take advantage of a girl. In particular if a girl was working at some place that offered some discount or charged less in general, I'd be happy to tip to bring it up to fox den rate territory assuming the experience was decent. Obviously at deep discounts there would be a wide range of tip you could choose based on quality of experience.
I kind of do the same thing when dining out, if the bill wasn't big but I had great service, I'll pretend the bill was big and base the 15% off of that assumption.