The Product Cost Amt. represents the actual cost of the Product(s) sold. It is set one of two ways:

For Sales Inventory Items, on transactions, that have not been allocated (just ordered) this comes from the 'Actual Cost' assigned on the Item Master File. This allows for calculating an estimated product cost, gross margin, and net margin.
For Sales Inventory Items, on transactions, that have been allocated this comes from the 'Actual Cost' assigned on the Lot. This allows for a more accurate product cost, gross margin, and net margin.

Note: Until the Lots have been closed for Costing, the Product Cost Amounts are subject to change.