So formula wise it would be something like this? d = dollars t = transactions count I(d,t) = Interchange total profit/cost to a bank from fees in and out F(d)

So formula wise it would be something like this? d = dollars t = transactions count I(d,t) = Interchange total profit/cost to a bank from fees in and out F(d) = scaling function for interchange returns based on cash pushed through as an arbitrary function G(t) = transaction cost function based upon arbitrary scaling. I(d,t) =F(d) - G(t)