Taxi drivers who bring travelers to Ezeiza are hoping to find passengers leaving Ezeiza for the city, only they're not allowed to get new passengers by the airport mafia to do so. Some get away with it. Just make sure it's a radio taxi. Make a note of the taxi number, the driver's name, agency phone number, etc. for your record.
This is an opportunity to make a deal with a driver, depending on your destination and how much you are willing to pay. It never hurts to try. Go to the end of the terminal where you'll catch an empty taxi to negotiate a price.
The current fares are still relatively inexpensive, especially for travelers with dollars or Euros in their pockets. At ten pesos to 1 dollar, the fare from Ezeiza to the city is still a bargain.
Why is it always Americans who are trying to find the cheapest of everything? They should have been here before 2001 when the dollar was equal to the peso.