When I started to punt regularly about 5 years ago. My sessions were normally 30 minutes.
The cost was $85:00.
I would give the WL 2x $50 notes. She would return to the room with $15:00 change.
I would just leave it lying where she could see it. I am sure that it worked as an "incentive" for good service.
It always worked and she always deserved the $15:00.
These days my sessions are normally 2 or 3 hours.
I do not always tip my regulars with cash.
I have other ways that they appreciate, Mouthwash, Baby Wipes, Energy Drinks, Phone Top Up Vouchers, etc.
I should add that I receive "Favours" in return.