Try using the OINM / OIVL ( depending on database version) table.
This should show the cost of the item as used as time of sale and can be linked to the AR Invoice lines (INV1) table on docentry. This will also depend on whether the salesperson recorded at the line level of the invoice is the same as that at the header level as these can be different
I am responding out of hours so don't have a database I can look at to give you field names and proper joins, sorry