If you are interested in staying with a family, there is a new project in La Cienega where you can do that. I have seen a sign on the road going through town, it seems that they have an office for it now.
Community Ecoturism Attraction in La Cienega
I have heard about an informal type of hotel in La Cienega, very basic, that has rooms that they rent out to people when they come there, but no contact information for it. Could be a cabana-like situation that you might not be interested in. You could show up and ask about it, then try one of the other places if the rooms are full.
The cheapest/nearest hotel would probably be Pontevedra or that place up on the hill above San Rafael. Next level up would be Quemaito or Playazul. Then there's Casa Bonita, which is quite expensive...
La Cienega is only about 20 minutes outside of Barahona, so you could easily stay there. The public transport situation isn't that bad if you won't have a vehicle. For Barahona hotels, look around in this section and there is a thread about the city of Barahona where several hotels are recommended.
Have a good trip!