I think it's a couple things.

1- as soon as you add "aircraft" to a part description, the price goes up. That little stamp on a part has to be backed up by a mountain of paperwork, thus some of the price increase.

2-economy of scale. How many FES battery packs will be made in a year? How many Tesla battery packs will be made in a year? We should know that the more pieces made in a given time frame drops the per piece price.

With Tesla building their own plant, they should have a better handle on manufacturing costs and quality. Time will tell though.