Good morning Clement and others who have requested an option for $0 cost. I'm now pleased to share with you that this is now available both in the admin and via the Inventory Item API.
In terms of historical cost on order line items, this is something the team are considering but not something we have a timeline for at this point. We'll keep you posted as we explore this further. Additional information about your use case for this data is always super useful in these discussions, so please share!