I am not sure what the total net is about illegals as a fiscal issue. They work very hard, and generally are not plugged into the social welfare system (and many forfeit for the moment social security benefits despite having paid FICA taxes), and tend to be young, and thus not plugged into the medical services mill. To the extent they make the economy more productive, that generates indirect taxes to the government beast. It is a hotly debated issue, and I don't think there is any grand unified creditable study that is at all persuasive, either way.
Frankly viewing the illegal alien issue, as a fiscal issue, for the government expenditure and revenue income statements, really misses the point. That aspect of the issue is rounding error, IMO.
You did read the threads here this week about the agreement, obtained under Freedom of information, that illegals who worked 18 quarters will be eligible for SS, vs us working 40 quarters?