For every dollar sent by migrants back to Mexico from the US, 25 dollars are spent in the American economy. Mexican migrants contribute to the American economy an estimated 65 billion dollars per year, contrasting with the 24 billion dollars that are sent back to Mexico.
A study done by the Universidad Autonoma de Zacatecas, states that Mexican migrants are subsidizing the Social Security system in the US, mainly because they cannot claim any benefits.
Parts of the economy where Mexicans contribute the most are construction, manufacturing, services, commerce and agriculture. "The real benefactors of the migration phenomenon are the receiving countries, in this case the United States gets more benefits from having the migrants than Mexico does" says Delgado, Dr. in Social Sciences from the University of Pennsylvania.

According to the study, Mexicans' contribution to the GDP has increased yearly for the last 15 years.

"In 2007, Mexican workers contributed with 484.8 billion dollars, a number that represents 3.7% of the GDP for that year in the United States. This number is lower than the total representation of Mexicans in the workforce, which is 4.7%"

Additionally, Mexicans in the US earned a total of 131 billion dollars, during that same year, 24 billion dollars were sent back to Mexico.

According to estimates by the US government, there are 12 million Mexican migrants in the US of which 7 million are illegal. Out of that number only 48% send money back to Mexico.

Personal note: One more thing to consider is that the money one person sends back to Mexico is used to sustain a family of 3-4 people, and they are able to get a better education and will therefore be less likely to have a need to migrate if they get good jobs in Mexico.