Privately owned air-conditioned buses are the cheapest way to get around the country. Be forewarned that the bus-music will be Dominican-loud and the a/c, if the bus has it, will be cranked up to the max.
Frequent service from Santo Domingo to the town of La Romana is provided by Express Bus. Buses depart from Ravelo Street in front of Enriquillo Park every hour on the hour from 5 AM to 9 PM; the schedule is exactly the same from La Romana, where they leave from Camino Avenue. In Santo Domingo, there's no office and no phone, but a ticket-taker will take your $4 just before departure. There's general chaos, a crazy kind of congestion (allow time in a taxi), horns blowing, and diesel fumes, but it all comes together. Also, watch your back, or rather your wallet and purse. Travel time is about 1¾ hours, and if luck is with you, the bus may show a first-rate American movie. Once in La Romana, you can take a taxi from the bus stop to Casa de Campo ($18) or Sunscape Casa del Mar ($25) or Iberostar Dominicus ($30).