If the number of illegal immigrants booked into the jail rises 10 percent and each inmate serves 20 days, it would cost taxpayers an extra $3.8 million annually.
If the departure of many illegal immigrants causes crime to drop and jail bookings of them decline by 5 percent, taxpayers would save about $2.4 million, based on the current average jail term of 25 days.
Breaking the new law is a misdemeanor, so it seems unlikely to have a sizable effect on state prisons, which typically house felons. A 5 percent change in the 6,100 illegal immigrants in state prisons, for example, could cost or save taxpayers about $6.6 million.
Any cost changes figure to be dwarfed by the amount Arizona is underfunded by Congress each year for incarcerating illegal immigrants who commit felonies or are repeat offenders, state officials say.
The State Criminal Alien Assistance Program provided Arizona and its counties $19 million in fiscal 2009, according to the U.S. Justice Department. There are estimates that the Arizona Department of Corrections alone spends at least $130 million imprisoning illegal immigrants annually.