For comparison- the cost of cheap labor

Lets take a look at two workers- one an illegal immigrant, one a legal one. They both have a wife and two kids, and live in a 2 bedroom apartment in the same neighborhood.

Mr Legal makes $18 an hour in construction as an independent contractor and pays his taxes.
Mr Illegal makes $8 an hour as an "undocumented" worker, working under the table.

Mr Legal makes $37,440 if he works 40 hours a week, 52 weeks a year.
Mr Illegal makes $16,640 under the same conditions.

Mr Legal then pays $4643 in Social Security and Medicare taxes, leaving him $32,797.
Mr Illegal, working under the table, pays nothing and still has $16,640.

Mr Legal takes the standard deductions, and pays $1,408 in Federal income taxes, leaving him $31,389.
Mr Illegal, working under the table, pays nothing and still has $16,640.

Mr. Legal pays medical and dental insurance with limited coverage for his family at $600.00 per month, or $7,200.00 per year.  He now has $24,189.
Between using the Emergency Room and EMS ambulances without paying, the county health department, and other state funded clinics at no cost, Mr Illegal pays nothing for health care, and has $16,640.

Mr. Legal makes too much money and is not eligible for food stamps or welfare, so he pays $500.00 per month for food for his family, or $6,000.00 per year. He now has $18,189 left.
Mr Illegal, having no documented income, and is thus eligible for food stamps, and his children are also eligible for free school lunch and breakfast programs. They are well fed at a cost to themselves of $0, leaving him with $16,640. (under the Federal "indigence" exception, illegal immigrants who would go hungry or homeless without assistance can qualify for food stamps and welfare. Since our illegal in this example has no reported income, his family would qualify for this exemption. it is highly unlikely that this reporting provision will have any immigration consequences for the immigrant)

Mr Legal rents a 3 bedroom apartment for his family at a cost of $13,200 a year. He now has $4,989 left.
Mr Illegal lives in a rent subsidized apartment, and only pays $8,400 a year. He is left with $8,240.

 Mr Legal must have car insurance at $1,600 a year, plus an additional $300 in uninsured motorist coverage, to protect his car and family should he be hit in an accident by Mr Illegal. He is now left with $3,089.
Mr Illegal doesn't have insurance. He still has $8,240. *(as a side note I run on at least 2 car accidents a week where the at fault party is an illegal. They never show up to court, and never have insurance. This leaves Mr Legal to pay the bill, meaning that he better have uninsured motorist coverage, or he is gonna pay.)

They still need to pay utilities, gasoline, clothing, and other expenses.

Mr Legal has $3,089 left, has paid a total of $6,051 in taxes and has gotten $0 in Government subsidies and money, for a net contribution of $6,051.

Mr Illegal has $8,240 left, and has paid $0 in taxes  and has received $18,000 in subsidies and Government assistance, his take home income and benefits are the equivalent of $39,543 in pre-tax income. That's right- to get what Mr Illegal has, Mr Legal would have to get a $1 an hour raise.