From one of my economics guys:

Jonah,
I’m unconvinced about the net long term benefit of large scale immigration, but since I’m unconvinced for all the traditional reasons, I’ll let that comment stand.
What I’ve always thought was more interesting (as a graduate of a labor economics program) was the degree to which the growth rate of the underground economy as a portion of GDP increases with the influx of illegals. I mean, illegals are often paid below minimum wage and never carried on the books so how much of their effect on the economy is simply not being recorded? The productivity gains are there but the actual costs never show up or are categorized as some other expense by creative accounting on the part of small business holders.
( One might even ask how much of the pressure to hire illegals comes from the mandated increases in minimum wage, medical benefits, taxes and other related costs associated with an ‘actual’ on the books hire? )
The only realistic estimates of the ‘costs’ of illegal immigrants come from things like uninsured medical care and as we continually hear, these costs are skyrocketing.