Exploitation and oppression are not the same thing, according to Marx. Those who are more oppressed may produce less surplus value than those who are less oppressed. Take, for example, a male chauvinist worker who beats his wife; she doesn't have a job because her jealous husband doesn't allow her to go outside & work for wages (he fears that she will flirt with her co-workers or something like that). She is clearly more oppressed than he is. In fact, he directly oppresses her, becoming an instrument of sexist oppression. Nevertheless, he produces surplus value, while she doesn't (though both of them belong to the working class). Another example. Compare employed & unemployed workers. Unemployed workers tend to be more oppressed than employed workers of the same stratum within the same social formation, but it is the latter, not the former, who produce surplus value.
Are workers in Mexico on the average more oppressed than workers in the USA? Clearly yes. They are more politically oppressed, they experience more economic hardships, etc. Do workers in Mexico produce more surplus value than workers in America? The second question is different from the first one. More oppression doesn't necessarily equal more exploitation & more production of surplus value, unless you argue that higher real wages = less exploitation = less production of surplus value, which Marx didn't. Marx thought, instead: "A rapid growth of capital is synonymous with a rapid growth of profits. *Profits can grow rapidly only when the price of labor -- the relative wages -- decrease just as rapidly.* Relative wages may fall, although real wages rise simultaneously with nominal wages, with the money value of labor, provided only that the real wage does not rise in the same proportion as the profit. If, for instance, in good business years wages rise 5 per cent, while profits rise 30 per cent, the proportional, the relative wage has not increased, but decreased" (emphasis added, Karl Marx, _Wage Labour and Capital_ at <http://www.marxists.org/archive/marx/works/1840/wage-lab/ch08.htm>).
Yoshie