Actually, the direct cost of welfare is the least of the damage illegal aliens do to our country, especially since PRWORA. By driving down low end of wage scale, illegals make it harder for low-skilled Americans to earn a living. You can bash highschool dropouts, but they are fellow Americans and when they can't raise a family WE have to deal with the crime, illegitimacy and squalor.
A surplus of illegals willing to work for low-wages and no benefits actually creates the demand for more low-wage workers. Businesses have no incentive to modernize. Instead of buying a new machine, you throw a couple more wetbacks on the production line. This hurts productivity.