When I know the price list and menu, I go in with enough for a $20 tip. In situations where the room fees and menu are unknown, I usually bring with me enough for the wished for services, plus a $20 tip. So suppose I am checking out a new place and am in the mood for a BS, I go in looking for between 30 - 45 minutes expecting between $40 to $60. I expect the BS to be between $60 to $80. I then pick the lower of the room fee, and the higher of the extra service, in this case that adds up to $120. I then add a $20 tip, so I go in with $140. I then adjust based on actual room fees, the attendant I happen to get.
If the 40/45 minute room fee is $50 I take that, if it's $60 for 45 minutes I take the half hour. When it comes time to negotiate the extras, if I get a good deal, and the service is good, they get whatever is left as tip. If the extras are expensive, then they can't take advantage of me in my weakened condition.
If need be, I have no issue with telling the attendant that I only have $x with me. They can then decide if they want to make a deal. Sometimes they say OK, sometimes they make you take the "cheaper" extra ... if the service is good, and I want to repeat, then next time I take enough to cover the "advertised" cost plus tip (and maybe extra if I figured she deserved more on my last visit) ... if the service isn't good enough to repeat, then I probably paid them too much even with the "deal" and lack of tip.