There is no set formula. I have two $50 to plow lots, and the salt prices are very different. The first is a very small lot, and very tricky to plow. It is however, very easy to salt. The second lot is a wide open rectangle, with very no obstacles. Plowing is very easy. The second lot has a higher price for salt, because it takes a good bit more than the first.
I do not discount for plowing and salting in the same trip. I lost a job this season because of this.
RB- Who is selling salt in Pittsburgh for $32.50/ton and $120/pallet? The lowest per ton price I can get is $50+