Hi
You have daily service from MDJB (Santo Domingo local Airport) to PAP, Haiti via CaribAir & Vol Air - both fly twice a day/ From SDQ Int Airport, you have PAWA providing 3 or 4 times a week service.
In Santiago, there used to be Caribintur Air that flew daily to PAP however I am not sure if they still provide service. You can call the airport in Santiago and check. Most roundtrip fares are US$250 to US$300 including taxes.
Hope that helps...