There are taxis assigned to the village of Benahavis, you will be able to get one at the number below.
Some people in the village also do airport runs or taxi services though, so check around.
A taxi from San Pedro de Alcantara should cost you about 17 euros during the day, and about 20 euros at nighttime.
952 86 90 90
or visit http://taxibenahavis.com/
The busstop is at the entrance of the village on the left.
There is a bus at 07.15 and one at 19.30 leaving Benahavis for San Pedro and the centre of Marbella.
Buses leave San Pedro bus station for Benahavis at 06.50 and 19.00.
There is a cheap direct bus service from the Marbella bus station to the Malaga airport which will get you there in 45 minutes (between 6 and 8 euros depending on the time, 1 euro extra with avanzabus online, but then you are certain to have a seat)
The busstop at Malaga airport is 20m to the right when you come out of arrivals.
Times (please check www.avanzabus.com just in case):
Marbella-Malaga airport 07.15, 8.15, 09.15, 10.15, 11.30, 13.15, 15.15, 15.30, 17.00, 19.15, 21.15
Malaga airport-Marbella 08.15, 10.15, 11.30, 12.00, 12.30, 14.15, 16.15, 18.00, 20.15, 22.30