I never understood why a man would walk into a club and pay a woman any amount of money to have her bump and grind on his lap for 3 minutes. If she is smart and seasoned it will be atop your knees and she wont be sitting anywhere near your crotch. Unless you "buy" more dances or tip her upfront. But a lot of clubs are strict, watch the clocks and you will actually be stopped and told to buy another ticket right then and there if you want it to continue.
If she has just begun and doesn't know any better you may be able to entice her to do all sorts of things because she THINKS that you will tip her.
And for those of you who think that the 20$ goes to the girl, think again. Most places keep at least half the money.