Answer
This points to migrating into a container that isn't meant to be populated by direct migration at all — Inventory Part unit cost values are meant to be determined by the application's own cost roll-up calculation process, not set directly via a migration job into the unit-cost container; if you hit this specific error migrating into unit-cost data, treat it as a signal to run the proper cost roll-up calculation through the application instead of continuing to debug the migration job's configuration.