Depends when you go.
Starting in june there will be opening of express route Romana - Bavaro/Punta Cana.
Before that route opens, you can take expreso Romana - Higuey (SITRAIHR) and then exoreso Higuey - Bavaro/Punta Cana (SITRABAPU). Romana bus leaves you in front of Bavaro bus terrminal. Price roughly 200 pesos total.