If you are referring to La Romana Airport, then go to www.dominicantaxi.com, excellent service, I have used them several times, very reliable and competitive rates.
Cost us $100US for a taxi(minivan) from La Romana aiport to Santo Domingo (Piantini?), we were 4, but the price is the same for up to 5 passangers. Our driver was from Taxi RS, do a search on Google for Dominican Taxi or Taxi RS.
If you have escaped the tourist trap of an airport at La Romana
There are buses with air conditioning that leave La Romana every 15 or 20 minutes for Santo Domingo. Any local Taxi driver can take you there for a couple of bucks. Whoe trip should cost less than RD$800.