> that would amount to $15 billion for all homeless in the US
We are already spending, as a nation, much more than this. We spend roughly $3B federally directly on homelessness (not including other programs aimed at poverty that provide assistance). California spends $1B. California localities spend $3B. New York City spends $3B alone.
These are just governmental expenditures, aimed directly at homelessness, and ignore charitable efforts.
Then I wonder how direct that spending really is. It doesn't seem to involve handing out money to homeless directly... Does it just end up in a general budget that's labeled "homeless", but actually gets used for all kinds of projects?
A lot of it used to be direct cash assistance to the homeless. This did not work well. The trend has been towards providing services.
One caveat with the analysis is that your census of homeless people does not count the people housed as a result of these programs. It also doesn't count the number of people that would have to be served that are not presently considered homeless.
Of course spending the money more efficiently would be preferable, but additional resources spent the same way would also work.