This is the paragraph immediately following the sentence you quoted:
> The impact on everyone else, though, is surprisingly positive. Giovanni Peri, an economist at the University of California, Davis, has written a series of influential papers comparing the labor markets in states with high immigration levels to those with low ones. He concluded that undocumented workers do not compete with skilled laborers — instead, they complement them. Economies, as Adam Smith argued in “Wealth of Nations,” work best when workers become specialized and divide up tasks among themselves. Pedro Chan’s ability to take care of routine tasks on a work site allows carpenters and electricians to focus on what they do best. In states with more undocumented immigrants, Peri said, skilled workers made more money and worked more hours; the economy’s productivity grew. From 1990 to 2007, undocumented workers increased legal workers’ pay in complementary jobs by up to 10 percent.
Don't make it look one-sided.
Besides, you didn't tell me what you think a ditch digger should earn, which is what I asked. But that's fine, because you can't tell me, because you don't know. In fact, nobody knows. It is an unanswerable question. Much like determining what someone's "fair share" of taxes or benefits is, or their fair standard of living, it is impossible to determine the income they "deserve." There is no absolute standard for any of these things. The only way to know if someone is being fairly compensated is to compare their income to some kind of absolute standard -- but such a standard does not exist.
In the case of unskilled American workers, maybe their incomes are 0.4-7.4% lower than they would have been in the absence of immigrants. But that by itself does not tell us anything about fairness. Perhaps they were overpaid before the immigrants showed up -- after all, there were other people willing to do the job for less money, so what did the American workers have to offer that justified a higher salary for doing the same work? Again, in the absence of an absolute standard of what their income ought to be, it is impossible to know if they are fairly paid, underpaid, or overpaid.