This (negotiating price) is actually quite common in the US especially if you do not have dental insurance. Many dentists will charge you much less if you are paying the cost out of pocket.
On a sidenote, I would suggest thinking twice before having dental work done on a vacation, of course if an emergency that's one thing but to travel just for cheaper dental work, not always a good thing. My aunt who is retired in Arizona has gone to Mexico to save $ on dental work only to have complications that no dentists in the states would touch forcing her to return after several days in much discomfort to Mexico. Her situation had nothing to do with using a sub-standard dentist in Mexico, anything can have complications.