I think you go Reporting > Analytics > Then set if you are affiliate or a publisher and work it out from the amount you sold divided by the amount that have been returned.
If the product is not your product, and you want to see how much money is being made per sale after refunds, then I always recommend going to: http://www.cbtrends.com/clickbank-products-performance-trends.php and enter the vendor name in the box. You can get some great stats and there is an explanation to the meaning of the slopes on the graphs under each one. Have fun