I think <PERSON>'s answer is fantastic, but I just wanted to observe one little fact: the energy that we harvest from nuclear fission reactors comes from somewhere. It isn't magic. When you start with hydrogen and create heavier elements, you are doing fusion, which is the opposite of fission. And that means that the energy released during fission came from fusion. Now, we can't fission elements down to hydrogen, because they are too stable. In fact, most of the binding energy inside elementary nuclei is not available to us for consumption. But it all got put into elementary hydrogen by someone, somewhere, and that "someone" was generally a supernova (dying star).
So given that we currently power a decent portion of our society by breaking apart elements, you can begin to imagine the power cost of assembling those elements from scratch. As o.m. observes, it literally takes the power of a star, and a violently exploding one to get anything heavier than iron. 435
A nuclear power plant runs on a surprisingly small mass of radionuclides. Current reactors produce on the order of 1,250 GJ/kg of enriched uranium. Which means that if we could run the reaction backwards and perfectly, we would need to supply 1,250 GJ to turn a kg of strontium back into uranium.
Now, uranium is much heavier than the elements in food, so this is not an apples-to-apples comparison, but strontium itself has a high internal binding energy, so I would guess that the order of magnitude isn't that far off from H->Ca. A kg of bread is about 2.5 loaves. Also, a kg provides roughly enough calories to feed a single person for a day. A GWe nuclear power plant consumes about 25 tonnes of enriched uranium per year. If we could instead perfectly transmute a kg of enriched uranium into a kg of bread, your billion dollar nuclear power plant could feed a grand total of less than 70 people per year. It costs about 140 million USD/year to fuel and operate such a plant, so you're feeding those people at the bargain basement price of about $2 million/person.
Needless to say, if you can afford to spend $2 million/person on food, you're better off building greenhouses. ;) 435