This website give information about taxi fares from the airport:

Official web page for Spanish airports

Although is not very frequent, some dishonest taxi drivers might try to charge extra money from tourists. Just in case, make sure that the meter is on when you start the journey and that it is set to 0 (zero), all supplements must be applied at the end of the journey. Should the meter not be set to zero at the start, require explanations and request it to be set to zero, failure to do this should result in taking another taxi.

The fare indicated at the end of the trip + all the authorized supplements (clearly indicated in the rear doors of the car) should be the final price.

The journey from the airport to the city must cost around 25/30€. If unsure of the fare:

  1. CONTRACT: ask the driver how much it will be, it is much more difficult for them to trick you into anything.
  2. YOU KNOW MADRID: most drivers will ask you if you know Madrid (question: "Conoce Madrid?"), the best answer you can give is something like "Si, amo Madrid!", give them an idea you aren't the new guy in town, they'll be more careful.

If they trick you into a dishonest fare:

  1. RECEIPT: ask for a detailed receipt detailing all the fares, extras and supplements.
  2. NUMBER: take down the identification number and/or the plate of the car, both of them indicated on the taxi door.

You will probably be refunded from the Oficina Municipal del Taxi .

2014 update: near the exits from the terminals there are information points regarding the taxi. You can find out there how much the trip will cost. For example, a trip within Calle 30 costs 30 € (fixed price) + 5,50€ suplement.

To take the taxi you just have to go to the taxi rank and an employee from Oficina municipal del taxi will help you out.   

 Visually compare different travel options to the center

You can also check most travel options from the airport to the city, visually compared by price, travel time, frequency etc.. . Just follow the link and enter the airport name:

http://www.cabmix.com/airport-to-city/