State Criminal Alien Assistance Program (SCAAP) is an initiative that aims to reduce the cost of housing immigrant detainees to local taxpayers. Counties apply through a Grants Management System, the appropriation for Financial Year 2005 is $300 million.
Per-detainee costs are not available (and would vary in areas depending on overall funding and inmate numbers), however you can see the amounts that were paid to applicants in FY 2004 (Bureau of Justice Assistance).
The Brownsville Herald gives some examples of figures for Cameron County, which lists the per-detainee costs in that county as $37 a day (Robeldo, D., County taxpayers shoulder burden of housing undocumented immigrants, Brownsville Herald, December 21 2003).
The most recent survey of inmates was in 1991, and states that 4% of all state prison inmates were immigrants, and most were arrested on drug offences (1991 Survey of State Prison Inmates, Bureau of Justice Assistance)