Regarding a new Pew poll (full poll results here):
[A] plurality of Mexicans think that life is better across the border, and a third want to move to America.
In addition, the Pew survey found that “two-thirds are dissatisfied with the way things are going in Mexico today” while “only 30% are satisfied with the country’s direction.” That may be why “a plurality of Mexicans (44%) believe life is better north of the border for those who migrated from Mexico” and “roughly a third (34%) still say they would move to the U.S. if they had the opportunity, including 17% of Mexicans who would do so” illegally.
Mexico currently has about 120 million people. Without looking into the poll’s methodology, 1/3 would be 40 million people.