Using vacant urban land to farm on is a great idea and we should do it now. It would bring food and people closer in ideology and distance.
As good an idea as it is, don't tell it to anyone who makes money in real estate, which includes almost every politician and all other people earning over $200,000 a year.
Yes, you can make money farming, but you can make a lot more money gouging rent on buildings built on arable land. Ask any New Yorker where a thousand dollars a month will get you. It will get you laughed at by any real estate agent you visit.
The instant the housing market recovers, city farms will be gone faster than that Pawn Stars guy screws you out of money.