An informal study looking at the urban farming yields, by Mara Gittleman. 67 gardens, with a total area of 1.7 acres in NYC generated 87,000 lbs of food, with a market value of roughly $200k in 2010. This is equivalent to about $3/square foot. Just looking at the financial aspect, if we’re talking about land which could be developed, the net present value, discounting at 5%, of $3/sq ft, is (even if we go out 100 years) only about $60/sq ft. If you build a 5 story building, then property values need only be greater than $12/sq ft for the urban farming not to make (economic) sense, and I’m going to go out on a limb, and guess that property values in most of NYC are, um, substantially higher than $12/sq ft.