Most of the cost is administrative. They have to have some government bureaucracy administer them. Otherwise, there's no interest.
I live where everyone grows and/or barters for their own veggies-and eggs and chicken when available-so I just have to wonder what would happen if the bureaucracy just got out of the urban farm business and charities like church groups taught the “homeless” to plant, care for and harvest the crops-thereby feeding themselves and doing something useful. I guess that would just be too logical and make too much sense...