Dollar and the Peso
These last few months have been nothing but good news for US travelers going to Mexico. Mexico tours are a better deal than ever before and the reason is not only the lovely, safe and delightful Mexican interior. The peso has slipped consistently in the last 6 months and so you save about 20-25% on your trip there.
Recently in the México City airport (MEX) I exchanged some dollars for pesos. The rate at the airport is better and the place is safer than other places in Mexico City. I actually changed all I had since the rate was so good. I did not mind returning to the USA with a few extra pesos.
I recommend the airport for changing money at the beginning of your Mexico tour. I also encourage you to walk around a bit and look for the best rate. The ones close to the gates are not as good a rate. You can make a few dollars by just walking 100 meters or so away from the gate and there are perhaps 50 exchange booths for you to choose from.
Also–watch for pickpockets and bag snatchers in the airport. If you are careful, no prob–but they are around and it is a thriving business there.