Every change is saved as a version with an effective date — each order is priced by the version effective on its day, and past months re-compute identically forever. With a cost the fixed supplier price = 60 % × net + 40 % × cost (his usual share plus our 40 % of the manufacturing cost — he gets his cost back and 60 % of the true margin, we keep 40 % of the true margin); without a cost it stays 60 % × net. The supplier price is always overridable by hand.