When they first started one guy tried charging me 7 bucks from the border to the Zona. That didn't work. No trouble since. They all start with the meter at 5 or 6 pesos. A ride to the border or back from the Zona should be around 20 to 30 pesos, roughly 2 to 3 bucks.

Don't let them tell you the meter is broken either, just get another one if they pull that.

Only occasionally.  I once had a guy who kept fiddling with the settings on the meter.  I was never sure what he was trying to do, but I suspected he was trying to change it from day to night.  I once had a guy tell me the meter was broken, so I told him what the it usually cost be X dollars in a taxi libre and that is what I’d pay him.  He agreed.  The biggest problem is with drivers getting lost or taking a round about route to the destination.  Overall I prefer a taxi libre to a yellow cab.