You can take a cab to Higuey and hop on a bus from there.
Maybe some poster here can tell exactly where in Higuey. The buses to SD are really cheap and would take around 4 to 5 hours to get to SD.
Just a thought.
I've taken that bus ride(s).
It was fun. But that's because I only had a backpck and had an entire day to kill. On boarding I noticed plastic milk crates stacked on the luggage racks (???). Again, I learned a thing or two about the "Dominican Way". After all the seats filled up (halfway through the trip at stop #12) the driver would count the number of embarking travelers at each stop and place one milk crate in the aisle for each boarding passenger.
It was a big, comfortable bus (to Higuey) but the overmax capacity sucked all the air out of the bus making it necessary to open the windows. There's nothing like riding a bus with some complete stranger's head resting on your thigh (I'm not the touchy-feely type) and when we got to Higuey we got off the big bus and got onto a smaller bus (after awhile) which was small (think school bus), crowded, had no AC and smelled like a gym locker. An hour later we got to PUJ and prior to the taxi to the hotel I went to the domestic terminal and arranged the return flight a couple days later.
Having travelled both, I highly recommend the domestic flight.