NAFTA has definitely benefited Mexico's economy, but its hard to say whether the average Mexican in Mexico has been benefited nearly as greatly. Due to large scale corruption in the government and by the drug cartels, a large portion of Mexican GDP is taken or given illictly, leaving out many of the average people.
I do believe that NAFTA has been of tremendous help to Mexico's economy. Today, Mexico ranks among the fastest growing economies in Latin America, as well as being a member of the G20. As a further sign of economic development, within a decade's time Mexico's birth rate will be lower than that of the United States. The growth of free trade over time has been inextricably linked to these measures of progress.
They are not making a lot of money, but the American corn might just be enough to supply their family with food. People in Mexico have not really been exceedingly profitable. Illegal immigration plays a large role in this. People that need to make a decent pay in Mexico struggle. They sell stuff like corn for more expense, because the government doesn't really cooperate with the farmers.