Urban land is much more expensive per unit. not only that, but you have the Capital expense of the machinery and buildings, plus the operational expense of heating, lighting, ventilation, and fungus control.
http://blog.zipgrow.com/indoor-hydroponic-farming-costs-prof... has a good overview of the costs. $110k for 500 square feet. excluding building. Then you have the opex for lighting which is $1100 a month. You'll need heating in winter, which will similar cost.
To put that in perspective, https://www.uklandandfarms.co.uk/rural-property-for-sale/eas... grade II soil. $80k, buys you 211266 square feet, with change left over for tractor, plough, seed and tillage hire/buy.
Moreover, its not an asset that will depreciate, unless one lets the soil blow away/don't improve it.
>> "What we found is that the two biggest costs in the indoor farm are labor and electricity"
this may be true but it doesn't mean the costs of outdoor farming are lower, plus the added cost of land (good farm land is expensive to own or rent)