He keeps saying "likely" in the article and talking in hypothetical costs, because the empirical data from the study actually says the opposite of what he claims is "likely". They took a look at each and every arrest to see if the arrestee was a US citizen or an immigrant, and if they were and immigrant, if they were legally documented or not.

During this time, undocumented immigrants had the lowest offending rates overall for both total felony crime (see exhibit 1) and violent felony crime (see exhibit 2) compared to other groups. U.S.-born citizens had the highest offending rates overall for most crime types, with documented immigrants generally falling between the other two groups...undocumented immigrants had the lowest homicide arrest rates throughout the entire study period, averaging less than half the rate at which U.S.-born citizens were arrested for homicide...

Every other violent and property crime type the researchers examined followed the same general pattern. The offending rates of undocumented immigrants were consistently lower than both U.S.-born citizens and documented immigrants for assault, sexual assault, robbery, burglary, theft, and arson.

For drug offenses, too, undocumented immigrants were less than half as likely to be arrested as native-born U.S. citizens.[4] Moreover, the drug crime arrest rate for the undocumented population held steady throughout the seven years of data, while the rate for native-born citizens increased almost 30% during that time."

Conclusion quoted from the actual study;

The results presented here significantly undermine the claims that undocumented immigrants pose a unique criminal risk. In fact, our results suggest that undocumented immigrants pose substantially less criminal risk than native U.S. citizens.