Each illegal alien cost US taxpayers a net $70,000 on average, for a low end estimate of $746.3 billion total.

That's lifetime and doesn't count future descendants. The estimates are low ball because they employ assumptions used by the National Academies of Sciences, Engineering, and Medicine that likely understate the long term fiscal drain, plus they use the old "11 million illegals" guess that most experts agree is long out of date. The Center for Immigration Studies opposes illegal immigration but I'm posting this to balance out the other thread featuring the ITEP "study". ITEP is a leftist lobbying group that I know from personal experience isn't credible at all. In debates over the years on Wikipedia talk pages, for example, it came to light that ITEP's tax numbers contradicted every reliable source (e.g. CBO, IRS, Tax Foundation, even the left leaning Tax Policy Center, all of which basically agreed with each other), and were in fact garbage without any corroboration. So here's the other side. At least CIS's fundamental position is logical:

The reason illegal immigrants are unambiguously a net fiscal drain is that less-educated people, native-born or immigrant, earn on average modest wages and as a result they tend to make modest tax contributions, while needing significant social services. As we pointed out in our prior study, research by the Center for Immigration Studies, the Pew Research Center, the Heritage Foundation, and others have all found that a very large share of illegal immigrants have relatively few years of schooling — most have not completed high school or have only a high school education. The fiscal drain illegal immigrants create is not because they are all lazy and on welfare, nor it simply because they often work off the books and don't pay taxes. Rather they tend to earn wages commensurate with their education levels and, as result, they typically have low incomes on average, though there are individual exceptions. Those with low incomes as a group, regardless of legal status, use more in public services than they pay in taxes. It's why cities and states worry so much about losing their middle- and upper-income tax base. It is middle- and upper-income residents who pay most of the taxes, which does not describe the average illegal immigrant.

https://cis.org/Report/Deportation-vs-Cost-Letting-Illegal-Immigrants-Stay

This jives with other research showing that non-citizens (half of whom are illegal) use more welfare than naturalized citizens, who in turn use more welfare than native born Americans.

https://www.washingtonexaminer.com/washington-secrets/census-confirms-63-percent-of-non-citizens-on-welfare-4-6-million-households

This is aside from illegal immigration's impact on suppressing Americans' wages, eroding law and order, and undermining the democratic social contract along with the very existence of the USA as an independent, free nation state.