Except, the cost of a substitute might, where the substitute participates in a real or regulated market. So, price under a rail monopoly may be bounded by the cost of trucking.
Price might be depressed below "whatever the market will bear" by trying to be a less attractive target to potential competition, or to fend off anti-trust enforcement. But those are not driven by your own costs.
Except, the cost of a substitute might, where the substitute participates in a real or regulated market. So, price under a rail monopoly may be bounded by the cost of trucking.