I guess you just have your own range in mind? Like I would now minimum give $60, for a few standard things that I expected to happen in the room.
Doing something extra would get more, preferably
kissing. I know some don't kiss and that's alright, but then I would not repeat as there are always new girls.
And if on top of that they do even more within the "semi-legit" limits (I don't really know what's the limit), then I would do to $100.
For repeating regularly, possibly stays at $80 tips. (I know I can get $100 for $80, hack, at some place probably $100 service can happen for $60)
May be I am over paying, or I am paying way too little. I don't know.
But judging from the reaction from new girls, $60 is an okay amount.