Well, the short answer is that you need to be prepared to invest enough money in capital and operating subsidies to plug the equity gap.
In our community land trust, 40% of the new homes we develop are either two- or three-bedroom homes intended for the families you're speaking of, who generally can't find an affordable place to live in private sector developments.
It's just not economic, so there needs to be an investment to plug that gap to make it economic and make it consistent with the mission of the non-profit or co-op developers.