There is an interesting report from the http://www.oecd.org/" rel="nofollow">Organisation for Economic Co-operation and Development which shows the inflows of foreign population by nationality into Mexico in 2011, the last year available. Apparently, Mexico is not a very popular destination for immigrants.
21,464 - Total foreign immigrants into Mexico in 2011
4,260 - Total from the United States
849 - Total from Canada
Just for comparison, in 2011 the United States accepted 1,062,040 persons as legal permanent residents. The data set does not list the outflows of foreign population by nationality from Mexico. In fact, it does not list the immigration outflows at all.
The data does list the total foreign-born population residing in Mexico in 2009, the last year available:
262,671 - Total foreign born population in Mexico in 2009
59,996 - Total born in the United States
10,869 - Total born in Canada
So, let's do some noodling on the numbers. If every year 4,260 US born persons immigrate to Mexico, and there are a total of 59,996 US born persons living in Mexico, then that represents a 14-year supply of US immigrants living in Mexico [59,996/4,260 = 14]. If the average American retiree retires at age 65, and the average life expectancy in the US is around 80 years, then retirees live about 15 years after they retire. That means the highest number of retired and still living Americans we would expect to see in Mexico is around 60,000. This is precisely what Mexico counts as Americans living in Mexico. Imagine that!
So where are the 1 million retired Americans that all the real estate promoters say are living in Mexico?
No where to be seen.