I go to Juan Dolio twice a year for 7 to 10 days. I always take a taxi from the airport and have never had a problem. All taxis charge the same rate and I have never been cheated. I don't remember exactly but think I paid $20 or $30.
When It's time to go from Juan Dolio back to the airport, I also take a taxi. There are several taxi stands in Juan Dolio and I usually make arrangements with one of the drivers the day before to pick me up at the hotel. The rates are posted and are regulated to protect the tourist. I have never had a problem with the time or the money. The taxi drivers have even provided me with their cell numbers in case I need to go anywhere else, like the capital.
I have used a taxi for the whole day, takes me to Sto Dgo, stays with me the whole day and returns me back to Juan Dolio for $80.
Things are nice In Juan Dolio. Enjoy the beach and mingle with the people. You will have a good time.