Always ask what the fare will be before you take a taxi. Taxi service from most Mexico airports to the city is often times double or more the normal rate due to airport monopoly (maybe $10 - $20 bucks extra per normal return fare from town).

Look for Colectivos (shuttle vans) that will usually have a much more reasonable rate into town. Some will carry their luggage outside the airport grounds and wait for a non-airport taxi or bus to come by for the savings.