Hello!
Such a complex issue as immigration usually isn't solved via a quick google! If only, eh?
Even if we put aside the numerous obstacles in attempting a "cost benefit" analysis of something as complex as immigration, what we are left with is still very contentious. Others have noted the slightly suspect nature of FAIR's independence (aren't all groups suspicious these days?) but even if we ignore that too, the calculations and methodology of the FAIR study have been widely refuted.
and
https://cei.org/sites/default/files/Alex%20Nowrasteh%20-%20WebMemo%20-%20A%20FAIR%20Criticism.pdf
Of particular note is the somewhat spurious manner in which the study purports to calculate the burden of the children of illegal immigrants.
Illegal immigration affects a number of people in a number of different ways. On the whole, there appears to be a rather large body of analysis that shows a net economic benefit to migration. Of course, such analyses are rather abstract and, I hear you say, "net benefit is of little solace to the individual who may have lost his or her job at the hands of an illegal immigrant."
Again, however, this argument is rather weak. Most credible studies indicate that the majority of blue collar jobs are under threat by automation rather than immigration (indeed, soon to be under threat are jobs of all collar colour).
But again, there are claims and counter claims around this argument too. And too, there are of course valid arguments other than the economic.
Overall, it's a complex issue. Yes, it is hard and perhaps stupid to say that illegal immigration is unequivocally a "good" thing, but that also holds true of saying it is unequivocally a "bad" thing. The issue has texture that is really not captured by a slightly dodgy think-tank doing slight dodgy calculation to come up with a slightly dodgy figure.