Perhaps you already do but...
...I think one of the best ways to reduce the "per shot" cost is to cast your own roundballs.
Next IMO is making your own lube and wads (If you use wads.).
Store bought roundballs go for ten to fifteen cents each (IIRC) and Thompsons Bore Lube is way too expensive for what you get. (I don't like the product so fifty cents a pound would be too much for my wallet.)
Some may disagree with me on the idea of saving money on bore lube because the ingrdients in some of the recipes can be quite high. Anhydrous lanolin, bees wax, and such can be pricey but if you use vegetable shortening and toilet donuts, you can keep the costs down.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson