What's the cost of homelessness? In Atlanta, it's $11 a day at a shelter, $53 a day in jail or $335 in a mental hospital. In San Francisco, make that $28 for the shelter, $94 for the jail and $1,278 for the hospital stay. What these raw numbers, compiled for the nonprofit Corporation for Supportive Housing, illustrate is that the societal costs of homelessness depend on the options we leave ourselves.

"While we know that nobody spends 365 days a year in a psychiatric hospital or in a jail ... what it shows is that being homeless can be very expensive," says Lyn Hikida, a spokeswoman for the California-based nonprofit that promotes permanent, low-cost housing for the homeless.

When it comes to serving the homeless, as with many things, the general rule is you have to spend money in one place to save it in another.

Homeless people tend to get in trouble a lot, be it for public urination, sleeping on park benches or pushing their belongings around in borrowed shopping carts. Since a night in jail costs many times more than a night in a shelter, it behooves government to keep homeless people out of the system.

That is why more than a dozen jurisdictions across the country have set up "homeless courts" that divert nonviolent offenders to services rather than cells.

"We want them to become productive members of society, and the homeless court program is designed to do that," says William Hoch, an Oklahoma City attorney who chairs the American Bar Association's commission on homelessness and poverty. "If they can't afford a place to live, then how do we expect them to pay a fine?"

But in the complex calculus of serving the homeless, the known costs must often be weighed against unknown benefits.

In a landmark study, researchers at the University of Pennsylvania tried to assess the cost-effectiveness of providing housing to New York's chronically homeless.

The study found that, before placement, a homeless person with severe mental illness used about $40,000 in services a year. Finding that person housing reduced those costs by more than $16,000, but the cost-of-living space raised net spending by about $1,000.

"Basically, providing housing to chronically homeless people with mental illness is a nearly break-even investment," says Dennis Culhane, one of the study's authors. "And we can either spend lots of public resources maintaining people in homelessness, or we can spend nearly the same amount and have much better quality of life for them, for us and a better outcome for society."