Geez, a study done by a group who's founder believed in eugenics, the "science" of breeding a better human race that was utterly discredited by the Nazis? Oh, and FAIR is a group that is devoted to reducing legal and illegal immigration. Oh, and before you criticize the below article, they are a libertarian org that advocates for smaller government, funded by Republicans...
"Its recent report, “The Fiscal Burden of Illegal Immigration on United States Taxpayers (2017)” by Matthew O’Brien, Spencer Raley, and Jack Martin, estimates that the net fiscal costs of illegal immigration to U.S. taxpayers is $116 billion. FAIR’s report reaches that conclusion by vastly overstating the costs of illegal immigration, undercounting the tax revenue they generate, inflating the number of illegal immigrants, counting millions of U.S. citizens as illegal immigrants, and by concocting a method of estimating the fiscal costs that is rejected by all economists who work on this subject...
...Merely using the correct numbers reduces FAIR’s estimated fiscal cost of illegal immigrants from $116 billion to $3.3 to $15.6 billion – and that is without touching their flawed static approach to counting how illegal immigrants impact the economy. This does not mean that the negative fiscal impact of illegal immigration is $3.3 to $15.6 billion annually, it merely means that using the correct numbers massively reduces their cost estimate. "
You can read the rest of why that study is bullshit in the linked article...
https://www.cato.org/blog/fairs-fiscal-burden-illegal-immigration-study-fatally-flawed