Sayeth the GAO:

Although DHS has not prepared official cost figures, USCIS officials estimated that a mandatory E‐​Verify program could cost a total of about $765 million for fiscal years 2009 through 2012 if only newly hired employees are queried through the program and about $838 million over the same 4‐​year period if both newly hired and current employees are queried. USCIS has estimated that it would need additional staff for a mandatory E‐​Verify program, but was not yet able to provide estimates for its staffing needs. SSA has estimated that implementation of a mandatory E‐​Verify program would cost a total of about $281 million and require hiring 700 new employees for a total of 2,325 additional workyears for fiscal years 2009 through 2013.