I can add that the part shows up in the "main assembly" CostingManager if it first is inserted into an separate assembly, which then is intersted into the "main assembly".
I have now been able to find out that the CostingManager will now calculate for any parts containing "C-Points" for routing. As soon as I remove those points, the part shows up with a cost in the CostingManager.
Is there some way to make this work? I need the "C-Points" in the parts I'm using.