US Dollar to Mexican Pesos? help!?

I bought pesos this afternoon, and the exchange rate was 13.07 to the dollar. Not only does the rate of exchange change every day, but it changes every hour. The bank teller told me that at 10:00 this morning the exchange rate was 13.18. However, these hourly fluctuations shouldn't mean much if your trying to figure the cost of a meal. Most foreigners use a 10 to 1 ratio when trying to figure the cost of their meal in dollars. And there is no such thing as a "typical" cost for a drink and a meal. Like any place else in the world it all depends on where you are in Mexico. i.e. A beer in Cancun will cost anywhere between $2.50 and $4.00 USD. That same beer in a small town twenty minutes from Cancun will cost you $.50.