If a purchase return order is created, and an item line is entered to return an item to a vendor, the standard purchase price (from either the purchase prices table or the last direct unit cost) is taken.

The unit cost doesn't change if the return order line is linked to an earlier purchase receipt by filling in the "Appl.-to Item Entry" field, if the costing method is not Specific.
But that would be the expected amount to be credited by the vendor, regardless of the costing method.
Also with other costing methods (except standard) the unit cost should be copied when an item entry is applied.