Hi Mitch,
APX stands for Optimization Extension Workbench, which is used to integrate external optimizers.
Since this is used to integrate external optimizers, which are designed by company itself or some by third party, i don't think workbench will cost you anything extra, but external optimizer will for sure.
Purpose
In the SAP APO system, you can supplement integrated standard optimizers with external optimizers that execute optimization for specific characteristics that are important in certain companies or industries (for example, dimensions, weight or temperature).
You have the following options:
- Connection of an external optimizer using Business Application Programming Interfaces (BAPIs).
- Integration of an external optimizer in the SAP APO system using the Optimization Extension Workbench (APX)
If you want to use an external optimizer that has already been designed by the manufacturer for connection to the SAP APO system, the interface configuration required is specified by the external optimizer. Proceed according to the specifications provided by the manufacturer of the external optimizer.
If you want to connect an external optimizer that has not been designed by the manufacturer for connection to an SAP APO system, you must have good knowledge of the following areas: SAP RFC and BAPI technology, Microsoft ActiveX and OCX technology, ABAP. You should also have a good knowledge of the SAP APO components to which the optimizer is to be connected.
For more information about integration and other aspects, please go through below link from SAP help.
Rgds
Sourabh