I recently read an article on the economic value of a community garden by Ngaio Hotte. This garden is located on the rooftop of a YWCA in Vancouver. The learning and interacting that occurs with these types of collaborative projects is known as "building social and knowledge capital" and has an actual dollar value.
The author of this article valued the service-learning, increased property value, volunteer effort, boost to corporate revenue at several thousand dollars. (For example, Employee Volunteer Programs have been show to boost company revenues by 0.5%.)
A community garden is worth much more than the harvested vegetables!
http://www.farmfolkcityfolk.ca/PDFs_&_Docs/MagazinePDF/FFCF%20Spring%202012.pdf