Hi Albert,
Cost per Unit is a stored KF. Without having it loaded / manually input via excel Dependent Demand Cost will not get calculated even if you have a Dependent Demand Qty.
In your example, you can define the cost per unit (e.g: you can load it, type a new value and save) in other periods but the calculation that is on top of the storage will always return zero if there is no Dependent Demand Qty. If you ask me, I would say this is ok as usually cost per unit is total cost / units produced. If there are no units, then there are no costs (making abstraction of the fixed costs).
Now coming to your latter question, it looks like your target is an optimization by minimizing the total costs. Then, Cost per Unit is not the right KF as it is not input into the Optimizer Supply Algorithm.
There are several cost KFs that are considered by the optimizer (e.g: non delivery cost rate, inventory hold or violation cost, transportation cost etc). You can find them all listed in the application help.
Supply Planning Optimizer - SAP Integrated Business Planning - SAP Library
Regards
Alecsandra