Ah, I see. Read my post above the one you quoted. How many dollars a particular currency is worth does not matter. A Japanese Yen is extremely cheap, 102.49052 yen to a US dollar today, ie 1/10 of a Peso, but their economy is the second biggest in the world, way ahead of Mexico.
If, (big IF) we were to get a North American currency, it would likely be valued at roughly the same as an American dollar and Mexican's would simply earn fewer 'dollars' than Americans and Canadians. The currency wouldn't suddenly have to collapse because Peso's aren't worth much.