Use the Inventory Price List in Reports. Of course, that only gives you list prices, not anything related to individual sales invoices.
The unit price is not currently exposed for custom reports, if that is what you are referring to.
A way to get the information you want is to create a Sales Invoice Totals by Item report. Then, drill down on the total for the item of interest. You will get the quantities and amounts for all line items for that inventory item. Export and do a little math in a spreadsheet.
You could get the same information in a custom report by querying for quantity and amount, but you would still be left to do the arithmetic in a spreadsheet. That approach, however, might get you closer to a better-formatted report.
The fact is, custom reports are not that good at dissecting individual line items in transactions.