I suspect the energy cost is a little higher than that because a commercial electricity customer (like Tesla) will often be subject to 'demand charges,' which are assessed based on the highest power used in a billing period. Since the point of a supercharger is to deliver a lot of power, the demand charge may be quite high.
I still agree with your conclusion though... Capital still dwarfs energy, even if they pay several times what you estimated.
I doubt that any individual super-charger station is a large enough load to be charged for their peak usage. It's a big load but nothing like a smelter or even a small factory.
Definitely more power than a small factory. A six station supercharger could be pulling 500 kW.
Up until 2012, demand charges from Souther California Edison kicked in at 20 kW at a rate of $17 per kW. One of the first 50 kW quick-chargers in LA county was shut down for months after the owner realized that operating it would tack on an extra $1000 to his monthly bill.
I still agree with your conclusion though... Capital still dwarfs energy, even if they pay several times what you estimated.