Depends. Generally with power there are elements of scale in favor of very large generation plants. Manufacturing does sometimes have their own power plants (aluminium needs a lot of power to refine) when you can cross that line. Also if the local utility is running into issues building large plants a medium sized plant might get under the regulations and so be cheaper. Of course when you run your own plant you also control the reliability - in some cases this can be worth a lot of extra cost. Utilities often give companies a large discount for having a backup generator that the utility can ask them to switch to at times of high demand: depending on how efficient Apple's generators are it could well be that it is cheaper to run them most days - utilities are going to be running peaking plants then anyway which are no more efficient and Apple saves the line loss price.

Generators are complex and weird. Many use more fuel at 50% of their capacity than at 80%, even though the latter is delivering substantially more power.