We use carjet.co.uk to book for when we go to Malaga. But like battyee says there can be extras, we are usually required to pay for full tank of petrol (rather than leave it back full) and if you're not doing much driving on the holiday then you don't use the full tank you had to pay for.
I rented a car from http://www.hirecarsinspain.com last year and found them good. They are an agent for http://www.malagacar.com/ who pick you up as you leave departures and bring you accross the road where you pick up your car (and so avoid the long queues downstairs in the airport).
Gulliver1,
Mind letting me know who you got the great quote from, and how it worked out? Any hidden extras etc?
Thanks,