In computing terms, conveyor belts everywhere cost O(warehouse space) to build, whereas mobile robots cost O(activity volume). Profits are also, most likely, O(activity volume).
As to energy costs, moving a shelf a at most 1/2 mile at low speeds on a level floor using electric motors probably uses less energy than you might think. Don't forget human pickers are say 150lb people walking up and down the same isles and they use about 150-200W to do so. The shelves weigh more (up to 1,000 lb), but you can a turn off most of the lights and electric motors are more efficient than walking. Assuming it's 400W a robot that's around 4 cents an hour per robot or something like 300$ a year.
