If your line item is set up as a sponsorship, you can tell whether it's costing you money by navigating to the Line Item detail page (you can do this by searching for the Line Item in the navigation bar). In the Line Item detail page, you'll see a 'Revenue Impact' field:
If the number is red and negative, it means that the LI exchange would have beaten the CPM you input into the sponsorship for the length of the line item. If the number is black and positive, it means that you are gaining money over our exchange based on the CPM input into the sponsorship (at a gross level).
To learn more about alternatives to selling on a sponsorship basis, reach out to your account team or read the following articles on the blog: