>1) No, land is not expensive. In agricultural areas, land is quite cheap. Organizing shelves of crops vertically solves for a problem that isn't actually a problem.
No it solves the problem of not having to truck food hundreds or thousands of miles, or ship food halfway around the world.
Cities are dense, most cities have less than a week of food in them (seriously, go to a grocery at odd hours, figure out when the trucks come, you'll see aisles with nearly bare shelves where the most common products have sold out sometimes 2-3x a week).
Vertical growing allows you to place food growing operations just on the edges of population centers, or even inside population centers. You can drastically cut down how far you have to move food. You can allow fruits and vegetables to get closer to ideal maturity before harvesting instead of picking early and artificially aging with gasses at distribution centers just before the product goes to stores. We throw away something like 1/4-1/3 of food in the United States depending on what data you look at. ONE QUARTER TO ONE THIRD of all food ends up in the trash, some of that is spoilage in the prodution and retail chain and some of it is neglect by the end user. Moving your growing operations much closer to the delivery point not only helps improve quality of the food and reduce damage & spoilage in transit but drastically reduces the amount of fossil fuels needed to transport food. The U.S. imports 50 percent of its fresh fruits, 20 percent of its fresh vegetables.
>The sun is free and electricity isn't cheap.
This is true and one current limitation to doing this. However getting people more open to nuclear, advancement of the PV industry and hopefully practical fusion in the near future will all help with this considerably.
>Shipping is cheap
Financially, not environmentally. See above... 50% of our fruit in the US is imported. That means it is picked well before being ripe, loaded into cargo containers and fumigated, shipped hundreds or many thousands of miles at which point some % is lost to spoilage or just damage, can sit at a port for days or weeks in the event of a strike, gets unloaded to truck or rail and shipped to regional distribution centers, is then either sold to factories or to retail chains and often artificially ripened via calcium carbonate or just flat out gassing it with ethylene, if going to retail it'll often get trucked to yet another distribution center before being trucked to the store and put on a shelf. That's insanely inefficient.
>In agricultural areas, land is quite cheap.
Compared to buying an acre in a downtown perhaps, farm land with decent soil, decent annual precipitation and water rights can be quite expensive as far as undeveloped land goes.