There’s a reason we should help the homeless off the streets and into housing. I’ll save you the usual moral arguments that you’ve all heard. Yes, there’s a duty to help the most needy and vulnerable, but it may be more effective for developers and advocates to talk dollars and cents when it comes to seeking resources and winning approvals for their affordable housing developments.

Here’s a figure: $41,000. That’s about how much a homeless person with mental illness costs the public each year, according to an analysis in New York City. Studies in other locales have found similar results when adding up the emergency care these people often need and consume.

In comparison, putting a person in housing costs about half of that, according to Sam Tsemberis, founder of Pathways to Housing, a pioneering Housing First program that moves homeless people off the streets of New York and into apartments. He said his program and similar programs in other big cities spend about $22,000 per client each year on housing and services, which is even cheaper than the cost of a shelter.

If money talks, these figures should shout from coast to coast. Affordable Housing Finance magazine has written about many developments built for the homeless over the years, but this month we take a closer look at the problem, especially the difficult, and often invisible, issue of family homelessness.

With so many families living on the edge, all it takes is an untimely car repair bill or a reduction in work hours for some families to end up on the streets, said one longtime shelter operator, noting that her waiting list for beds is the longest it has been in years. One of the people I talked to was a young mother of two in California whose world was turned upside down when she was laid off. She had worked for the company for 13 years. When I asked her why she was willing to step up and tell us her story, she said she wanted people to know the many faces of homelessness.

I also spoke to a father who lived in a shelter with his family for two months. He’s in an apartment now. He catches three buses each morning to get to school and work. To get back home in the evenings, he spends two more hours on three more buses.

As rents begin to creep up in many markets, finding and staying in housing will become harder for even more people. The answer to helping these families as well as the chronically homeless and others, however, isn’t a mystery.

I attended a National Alliance to End Homelessness (NAEH) conference in early 2006 that just happened to take place not long after several homeless men were beaten, one fatally, in Florida. During a brief exchange, I asked one of the key people there what it was going to take to end the attacks on the homeless. Without missing a beat, she answered that the solution was for them to have housing.

At the time, I was left a little puzzled and a little unsatisfied by her response. Now, I realize just how good an answer it was. It was so straightforward, so direct, so right on. It was perfect. Build more affordable housing.

We can either invest some money building housing or spend a lot more money paying for emergency services each year. When you have to make the case for affordable housing, use the numbers. They’re on your side.