1. Cost of Taxi about 100 euros each way
2. Airport bus Malaga Airport to Marbella bus stop 45 mins and about 5 euros each way see timetable on internet and then you take taxi (was about 30 odd) or local bus to near Duquesa http://www.gomarbella.com/marbella/malagabus.htm
3. Leave car at Malaga airport Brunoscar charges 19 Euros plus 1.5 per day. They also do car hire. We used them car hire for 2 years before we bought a car.
http://www.brunosparking.com/