If you go by public transport (minibuses) you can do it for under US$10, by making stops in Higuey, La Romana, San Pedro and then Boca Chica. If you take a taxi it will cost you about US$100-US$150. You could also fly from Punta Cana to Boca Chica (landing nearby at the Las Americas International Airport), and then taking a taxi to Boca Chica. This would cost less than the taxi from Punta Cana to Boca Chica.