Regardless, the relative price of food is usually a reasonable proxy for the amount of energy that went into its production. If the food from the urban farm is more expensive than equivalent food from an industrial farm, it probably took more energy to produce (and consequently probably released more greenhouse gases during its production).
I find your message confusing, when you say price, do you talk about final retail price or production cost? (fruits and vegetable are high margin products in food distribution)
When you talk about energy, do you take into account human labour? Natural ressources are considered free in our current economic system, that is why it is cheaper to grow food by damaging the amazonian forest with nasty coal/oil fed machine rather than doing local permaculture with more human beings and passive tools.
