[RTuesday]

The Telcel Amigo blackberry 15 day plan is 286 pesos (possibly a few % more if that's not a border area). It's limited to 1.5GB of data in that time.

You probably get 50 pesos of calling with your sim (which costs 200 pesos in this area, it does vary). So you'll need an immediate recharge.

In round figures, local calls are over 30c (US) a minute, and over a dollar a minute to the US, on the standard tariff. Incoming calls are free (when you are in the area you brought the SIM).