depending on how many persons and luggage the trip cost from 25-44 euros.
depending on where you land at Barajas airport you go to the taxi stand;not the individual taxis which charge more.
T4 lower level,arrivals
T1 lower lovel, arrivals, in front of gate 2.
T2 lower level,arrivals, in front of gate 5.
T3 lower level, arrivals, in front of gate 7.
from T4 the new terminal you take line 8 metro aeropuerto T4 the other terminals you leave from line 8 at Aeropuerto. both take you to ministerios then line 6 to Pacifico and then line 1 to Atocha train station.
if you insist on the taxi ,this is a service i use when traveling on business there
http://www.taxiflot.com/contactar.htmlin Spanish only