Technically, the meter should be turned on even if you've arranged a price with the driver. I used to do that with my taxi. If a taxi is pulled over by the police while taking a passenger to Ezeiza and the meter isn't running they can get into trouble. The exception is for the taxis that work at the airport. A coveted place for taxi drivers, which requires dealing with mafia in order to work there. If it's a mutually acceptable fare, why would you complain? Either way, you have every right to insisit they turn the meter on.