Hello Deepak
In OBYC Cost Difference should be Transaction PRD and that is affected by General modifier, Valuation class and valuation modifier (groupings of valuation areas)
I do not think that you need a substitution rule.
Is there any chance that you subcontract materials are different valuation class? and defined differently in OBYC? are they in their own valuation area?
Is the PPV generating on the 101 movement type?
regards
Althea