Given the room rates at the St. Regis, one of the most expensive places to stay in the city - I find it a bit humorous that you'd be trying to save US$5 or $10 on cab fares. Having said that, the hotel car rates - for very nice vehicles - for trips to/from the airport are most likely in the $350-400 peso rate. Divide that by 12.5 to find the approximate fare in US$. A 'Sitio 300' "authorized" taxi from the airport will cost you about $160 pesos. You can book a hotel or Sitio 300 car ahead of time, for a return trip to the airport. Return fares from the hotel with Sitio 300 will be less than what you were charged for the trip to the hotel when you arrive. I was in Mexico City last week for the fiestas patrias and accompanied by an elderly friend who'd not been in the city in decades. I was continuing on to another city in Mexico and he was returning home after a week, so I sent him to the airport in a hotel car from the Marriott on Paseo de la Reforma, for which ride the driver charged him $300 pesos. His safety was worth the money charged. Sometimes, when you try to cut corners there are unfortunate consequences. Have a great trip.