Besides the number of illegal aliens requiring services such as health care, emergency housing, food stamps, and other programs...
FYI, the greatest impact is on the cost of public education. As am example in California the annual per child cost of the public education system is a shade under $10K. The consequences of unregulated immigration now consumes about 1/3 of the public education budget which is the largest single program in the state budget exceeding $30B annually.
Oh yes, public education. I don't know how that slipped my mind. All that money that goes into it, and kids come out illiterate. Mexico has free education right through college, don't know about other Latin American countries though. It really galls me that illegals will get a better deal for community college than my kids will.