Generating and downloading an Orders report can give you detailed insights into how your business is doing.
To see the cost of a product in an order, just click on the Line Items tab at the bottom of the Excel window.
The Line Items tab shows every product sold over the defined time period.
By scrolling to the right, you can see the cost and profit of that product (both by unit and by line, if more than one unit is sold).
By summing the cost per line item column, you can get a COGS total for that time period.