I agree mostly with this approach, but with the caveat that it's not for everyone.
i negotiate high-dollar deals for a living, and have done so for over 25 years at this point, so more than 1/2 of my life has been spent studying the other side of the bargaining table for the best win-win and best goods for the best value. It takes a bundle of things to make that work: usually, if the parties get along, it's better (that could be mutual physical attraction, sexual chemistry or just the personalities clicking), or if it is unilateral (which is often more than not), making the other party think that they are getting more than their equal value (even if you were prepared to shell out more). I would agree that nickeling and diming a garoto for a difference in payment for services that basically amounts to only $5-$6 USD, to me, is absurd -- and would probably ruin the experience.
Most people are not going to be as experienced in the art of negotiation as I or Tomcal, though -- and a language barrier might make that task seem more daunting of an effort when you are not actually there to experience it. However, I am here to tell you that, once you are there, inside the sauna, it's really not all that challenging. You're not going to be wholesale ripped off in the sauna by garotos. They are mostly friendly, approachable and there to make money -- and if there is a communication gap between you two, trust me -- when it comes to the almighty dollar, they will help you come to an agreement with them or move on if they cannot.
I am pretty well-traveled and experienced in this scene in multiple countries. I have often invited some travel buddies who are less experienced (but not shy). All of them were quick learners at the saunas.