My wife and I just returned from a trip to Acapulco. We bought a couple of 50-Peso cards that were good for about 10 minutes each from a pay phone. About 45-cents a minute. Not the best rate, but competitive, and it was easy for the little bit of calling that we wanted to do. Got our cards at Wal-Mart and a local mart, but they were everywhere in town.
Don't call from the hotel room/timeshare/wherever you're staying! Higher cost.
Mike