Welcome to cruise-chat. It was over a year ago when I visited Caso de Campo, but there was a very well-organized taxi service at the port. When I was there it cost $10 for a mini-van taxi for 1-5 passengers for 1 hour that would take you anywhere you wanted to go. After the first hour, you negotiated with your driver for rate/time.
If you are in Caso de Campo for the day, I suggest you purchase a day pass to Caso de Campo. If you are there in the evening, do not miss the Kandela Show performed in the open-air amphitheater in Altos de Chavon Village.