question Nafta/Mexico

[Robert Went]

In a discussion about the consequences of trade liberalization a Dutch
economist who has worked before for the World Bank claimed that because of
increased corn imports in Mexico from the US (NAFTA) prices for basic food
(corn) for millions of poor Mexicans have been halved, so that - although as
a consequence many Mexican peasants have lost their jobs - this has been
good for the poor.
Suggestions for studies/websites to check/counter this claim are welcome.

Thanks,
Robert Went