The wages are lower but the Mexican cost of living hasn't been bid up by severe development restrictions, badly planned infrastructure, and finance industry corruption. Middle class life south of the border is more comfortable than in the USA because prices for housing and transportation aren't driven by status bidding wars for the limited set of safe, quality locations and good school districts.
Unskilled workers still make a lot more in the USA and have a richer life there, but the smaller Mexican educated class is maintaining its quality of life as it expands. It already matches the wealth north of the Rio Grande.
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I should add an example.
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For example, imagine we're comparing LA to Guadalajara or SF to Mexico City. With a degree from a good school, a civil servant might start out making $750 a month in Mexico or $2200 a month in the USA, after taxes. A decent small shared apartment will cost $200 in Mexico or $800 in the USA. The American will need a car for at least $400 a month, but cheap, efficient, and somewhat comfortable public transit is available all over Mexico. Electronics and internet cost the same. Splurging out at a nice restaurant can be done for $15 a person in Mexico that compares to $60 a person in LA or SF. The Mexican will graduate without any student loan debt while the American is paying every month, so that nice restaurant will be out of reach in El Norte, anyway.
A fairly well paid engineer in Mexico might be making $2200 a month while his American equivalent makes $7000, after taxes (think $100k-$140k US salary) . A nice apartment in a fancy part of Mexico City suitable for a family might cost $800 a month, but development policy has kept single family homes equally affordable in central areas, too. Nice apartments in SF cost $3500 a month and houses cost much more. Most professionals in urban Mexico don't need or want a car, but in SF the engineer and his spouse will both spend so much time in one that they'll need two nice cars and dedicated parking. Okay, I already can't figure out how the US person can afford to have a family at all; maybe that's why they don't. The Mexican engineer will be living pretty and thinking about a third kid. And Gosh help the American if he ever needs health care.
On the other hand, the minimum wage for unskilled labor in Mexico is about $5 a day.