From the Social Security Administration:

While unauthorized immigrants worked and contributed as much as $13 billion in payroll taxes to the OASDI [Social Security and Disability Insurance] program in 2010, only about $1 billion in benefit payments during 2010 are attributable to unauthorized work. Thus, we estimate that earnings by unauthorized immigrants result in a net positive effect on Social Security financial status generally, and that this effect contributed roughly $12 billion to the cash flow of the program for 2010. We estimate that future years will experience a continuation of this positive impact on the trust funds.