A cost typed in once, treated as the real cost.
The cost field on a product was right the day someone typed it. Since then there has been freight, duty, a supplier price rise, and a batch bought at a different price. The unit you sold on Tuesday came out of one of those batches.
Cost field on the product
$4.00
Typed in once, last year
Landed cost of the unit sold
$4.50
$4.00 from the supplier plus $0.50 of freight, from the oldest lot with units left