Sanya International Airport is a short distance from most hotels and residential areas, so getting there usually requires a taxi or bus service.
Refer to the list below for approximate travel times and fair taxi costs:

Sanya City: 10-15 minutes - Taxi Fare: 20-35 RMB

Dadonghai: 20-30 minutes - Taxi Fare: 35-45 RMB

Yalong Bay: 35-45 minutes - Taxi Fare: 60-70 RMB

NOTE: Sanya airport taxi drivers are notorious for overcharging from the airport: to get around this, head to the second deck (for departures) when you deboard in Sanya (head up the escalators outside the baggage claim.  There are usually rogue taxi drivers up there willing to bargain for a reasonable rate (50 RMB is more than enough to Dadonhai, for example).