raphster,
ricktoronto said:
"Oh, and when you sell them back at the end of your trip you will get about 1/2 of the real value."
No doubt true. If you enjoy your trip and plan to return, I would normally recommend you take any pesos remaining with you for next time. If you are the rare visitor who doesn't plan on coming back, spend any you have left on souveniers, etc. With the lousy exchange rate right now, you may want to do that anyway.
Because there are cambios everywhere, there is no need to exchange everything at one time. Convert dollars to pesos as you go, a couple of days worth at a time. That way you won't be "stuck" with a lot to unload at the end of your trip.