[Gerald]

Depends what type of massage you want and what she offers.
If you want a 15 min foot rub, then it probably won't cost you much.
If you want a full body, deep massage while she softly sings "Somewhere Over the Rainbow" then she may want a $300 tip, assuming she wants to sing (if you get my drift).
Best to just load up the old wallet then ask the lady what services she is offering, once you are in the room. She will appreciate the respect in asking rather than seeming to presume.
Remember the door fee is what she pays to use the room. The tip is what she takes home with her. If she gets nothing, you get nothing.