Best formula for price depending on amount
I think your expectation that a "best" formula exists is very unreasonable.

There is no "best" car design, there isn't a formula to design the fastest airplane with total weight $x$ and there is no easy formula that will give you the best price in general situations.

Sciences have a lot of unsolved optimization problems. Economics, psychology and most sciences involving humans have a lot of seemingly unknown variables, further complicating things. So while there are some principles which you can base your pricing strategy on I have never heard any reasonable person claim that a general best price mechanism exists and I don't think it does.