I'm going to assume that your screen name indicates that you are from HK.
All but the most rabid agree that illegal immigration costs the US taxpayers (there is no such thing as "government" money - it comes from you) a phenomenal amount in schooling, health care, health care, health care, imprisonment of criminals, unlicensed drivers, law enforcement, etc, etc, etc. that any small benefit to the overall economy by using unlawful labor. ( And I truly doubt that there is ANY benefit from that)
Did I mention health care?
Build the wall - enforce the law