On July 26, Steven Camarota of the Center for Immigration Studies testified before Congress, and his prepared testimony is in "Immigration's Impact on Public Coffers":

There is general agreement that the fiscal impact of immigration depends largely on the education level of the immigrants in question. Immigrants with a lot of education pay more in taxes than they use in services, while those with little education tend to have low incomes, pay relatively little in taxes and often use a good deal in public services. In the case of illegal alien, the vast majority have little education, and this is the key reason they create fiscal costs. Illegal families often receive benefits on behalf of their US-born children. As a general proposition, the large scale immigration of less-educated immigrants (legal or illegal) creates significant funding problems for social programs, including those for retirees, even though the immigrants work...

A detailed description of the costs follows. Obviously, these findings will be disputed. A much better way to deal with this issue is described here.