It would be better to place vertical farms inside the city in order to minimize the cost of postharvest procedures (less power needed for storing and transferring the products in fridge-vehicles etc. 

For more information check the book Plant Factory by Kozai.

For coastal areas check Sundrop farms in Australia, even though it is not vertical it is pretty much the same logic. 

Thanks for the reply.

That's what I had in mind for such spacious areas. I was looking at the desert forest project but Sundrop has nailed it! Those glasshouses must smell lovely!

It was the possibility of doing it in an urban environment, cost efficiently that drove the question as product generation would possibly be not be enough to cover the overheads.