This is one source I found just to show the cost of this to our system:
Of the 64,124 cases reported to the Commission in fiscal year 2023, 21,504 involved non-U.S. citizens. Non-U.S. citizens accounted for 33.7% of all individuals sentenced in fiscal year 2023.[1]
https://www.ussc.gov/research/quick-facts/federally-sentenced-non-us-citizens
We should also look at GITMO for the worst of the worst, or building a prison camp in Liberia.
One in five US prison inmates is a ‘criminal alien
https://thehill.com/opinion/immigration/407312-one-in-five-us-prison-inmates-is-a-criminal-alien/
In terms of cost, federal taxpayers shelled out more than $15 billion during the period studied — or $2.5 billion a year — to keep criminal aliens behind bars in federal, state and local facilities.
Many are repeat offenders. Of about 146,500 criminal aliens who finished a federal prison term, about one in six — around 24,800 — already had been imprisoned again at least once.
El Salvador knows how to deal with badasses. Make a deal with them to take these individuals into their prison system.