A controller for directing operation of an air pollution control (APC) system, requires a consumable to perform a process to control emissions of a pollutant. The process has multiple process parameters (MPPs). One or more of the MPPs is a controllable process parameters (CTPPs) and one of the MPPs is an amount of the pollutant (AOP) emitted by the system. A defined AOP value (AOPV) represents a limit on an actual value (AV) of the emitted AOP. An interface receives a value corresponding to a unit cost of the consumable. A control processor determines the cost of operating the system based on the received value corresponding to the unit cost of the consumable, and directs control of at least one of CTPPs based on the current value of that CTPP and the determined operating cost.