This is decided from the part used in this process. It is possible to choose which supplier one should create the external operation on, with the currency used for the specific supplier. 
On the operation for External Machining (or other external operations), there is a field: ‘PoWorkPart’ with an inspect icon behind it. Using this, you enter the defined part with class: WX that is used when ordering the external work from the purchase application (PUR), in the External Work folder. 
On this ‘part’, you can add purchase prices as you would on any other part ordered from the supplier and with the required currency.