Tex, can speak first hand to your question. Have done todo la noche with a few girls who never addressed money up front. This was after dinner, dancing and what is generally refered to as "date night". In the morning, would offer $70-$100 in colones as a "propina" or "regallo". On two occassions, the girls looked suprised and would ask for $150-$300, which, they would remind me, was their standard rate. I always stood firm and said that what I gave was a fair price. Actually told one who tried to stand her ground that "making love to her was like ph*cking my ex-wife and she should be happy I was in a generous mood because the dinero she got was much more than she was worth".

After an aukward silence, she left.
Chica's dont get loud and in your face like gringa's when their mad, they kinda sulk quietly and go away. Funny thing, saw the "ex-wife" girl my last trip. She smiled and wanted to get together again. Funny how quickly she got over the dinero thang
In retrospect, I think "no money mentioned up-front" is a sales tactic that some of them use to take advantage of a nice guy. Remember, Just offer whats fair and stand your ground. They will respect you more than if you whimp out and cough-up the extra dollars.
Papa Nut