Hi
Calahonda is approx half way between Marbella and Fuengirola, I would hazzard a guess the cost of a taxi would be approx €20-25.(one way)
The cheapest way is by bus and train, and the journey time maybe just over an hour.
You'd have to get the bus that runs from Marbella to Fuengirola and the link below shows the timetable, it's saying 1 hour 15 mins, but that's for the journey from Marbella to Fuengirola.....I reckon you'd be looking at approx 35mins or less.

From the central bus station in Fuengirola, you walk a very short distance to the train station...must take all of 3 minutes...200 metres away.
Here you get the train that runs every half hour from Fuengirola to Malaga...you can't go wrong, there is only one train. I don't know what time the bus arrives in Fuengirola, but the trains leave Fuengirola at 47 and 17 minutes past the hour, the time to Arroyo on the train approx 15mins.
You get a ticket to Arroyo/Benalmadena and as you come out of the station, you turn left and follow the road walking around and upwards.

http://www.gomarbella.com/costadelsolbuses/marbellabusstation.pdf

Sanji

Excellent advice from Sanji. I'd budget a bit extra on that taxi fare, however. I reckon as much as 10 euros more -30 to 35 euros from Calahonda.

Tony
Cheers, I don't "do" taxi's if I can help it.

Forgot to say that if you just miss a train in Fuengirola, you'll have to add another half hour onto the journey, waiting for the next one.....or go for a drink!

Sanji

Thanks all - great info! Jo