It's good to ask first, my third trip to Sosua I agreed to pay a barber 150 pesos for a haircut, when I got out of his chair, after a shave also, he demanded 700 pesos, I was peeved and gave him 600 pesos, later finding out the going rate is 100 pesos, inclusive....
Americans and Dominicans sometimes are expected to pay different prices. And I hate to have that heated discussion when i'm in the barbers chair getting a shave with a straight razor, which did happen to me once.