Field help (LINCSTPUR2) 

The line purchase cost corresponds to the amount totaling the stock cost and the costs that remain payable by the buyer and are not used for stock valuation (transportation cost, non-deductible taxes, etc.).
It cannot be modified and is displayed only if the transaction authorizes it. It is expressed in the currency of the company.

Note: the non-deductible tax amount is displayed in the 'Line cost’ window, accessed by right-click on the line.