i always try to negotiate before any money is handed over especially if i am trying out a new place,
its very simple to do, just be polite and straight to the point and ask what extras does the girls do and how much for the tip,
and if its doesn't fall into the tip pre limiters $20HJ/$50NHJ/$100FS, then ask her will she do it for this price, they will always want a tip.
if not walk away and go somewhere else.