Here's a Canadian article on urban agriculture in Philadelphia:http://www.cityfarmer.org/Phillyurbag9.html

"According to Libby J. Goldstein ... president, Food & Agriculture Task Force, Philly's 501 community vegetable gardens produced $1,948,633 worth of fruit and vegetables in 1994. A total of 2812 families (12,093 individuals) are involved in the vegetable gardens."

http://www.newvillage.net/Journal/Issue2/2urbanagriculture.html

"The truth is that profit margins are often very slim in agriculture, depending on what is grown. Shortening the distribution chain by growing food locally puts more money in the hands of producers while consumers pay the same for fresher, healthier food. As Mary Seton Corboy of Greensgrow in Philadelphia says, "If we sell our lettuce to a produce distribution center, they pay $5 for a 3-lb. case. If we sell to a middleman, he gives us $8.50 a case. If we sell it ourselves to the restaurants here in town, we get $13-15 a case. Part of the trick is to keep expenses to a minimum -- easy to do if you sell to local customers. Even so, Seton Corboy pinches every penny to make ends meet."

This page also has a good collection of resources on urban agriculture.