You have to use the value entry table.
There is a Unit Cost field on the sales invoice line table but this only gets populated when the item is added to the document. This field doesn't get subsequent adjustments/updated so it can be incorrect.
You can also look at the Statistics page if you only needed the cost per each invoice, as the Statistics page will have the updated cost. However, this isnt really useful in your case if you're looking to build a report.
