The last time I was there in May, 45 pesos or so was the standard flat rate from the city to the airport. You may pay the toll of 5-6 pesos as well. $19 as a flat rate would be okay, but a few dollars higher than it should be. My last time, I had him use the meter, and it was 2 pesos less than the flat rate, but still paid him the 45p.
If there is an problem, I'll just get another taxi. They love the airport runs, as they can often get a ride back, as well.
My simple rule for taxi drivers: If they don't agree to use the meter (or a fair flat rate), then I'll simply get out. Take a picture of the taxi meter unplugged along with his license number. About 50% of all the troubles I encounter in traveling are with taxis.
How were your DL flights?