a) Urban Farming commonly produces leafy greens and water rich produce like tomatoes and cucumbers. This is commonly known as 'horticulture'. The horticulture to 'farmland farming' ratio is already less than 1:10.
A larger part of horticulture are crop that grow pretty well on the fields and that are commonly not produced under glass in CES (controlled environment systems). These are crops like Kale, Cabbage, Beans, Peas, Potatoes, Carrots etc. The ratio of CES to horticulture is also well above 1:10. So in summary even if we'd take all the CES production into the cities, it would be well below 1%
b) Viewed from the other side, from the available area. Even in a relatively boring and unpopular city like for example Dortmund/Germany it is extremely hard to find available plots that would be suitable for UF. If the plot is in a residential zone the square meter price is 200-400€. Commercial zone: 80-100€. Agricultural (outside city boundaries): 3-5€.
So inner city plots are waaay more expensive than agricultural land.
We are currently trying to find roundabout three acres of inner city space for an urban farm. With the help of the municipality. It is going to become very hard to find something. To find anything...
Just watch The Jetsons :)