Update Inventory Part Purchase Price field {cost}

I have a script that updates a number of fields on Inventory Part records, but I am struggling to update the Purchase Price field succesfully.

Looking on the Record Browser it is listed as a 'currency2' field type (which is the same as the TransferPrice field).

My script is running these lines:

        item.setValue({fieldId:'transferprice',value: purchasePrice});

        item.setValue({fieldId:'cost',value: purchasePrice});

The Transfer Price field updates successfully, but the Purchase Price field does not. They are being populated with the same variable, so I cannot see why one updates and one doesn't.

In reading the help notes I see there is a peculiarity around the Purchase Price showing prices in the same currency as the Preferred Vendor field. Could this be why the field is not updating? Do I need to specify the currency in some way?