I have created a service and entered a cost against that service but I cannot see how to report the net profit for customers that bought that service

 Hello Dw77, thanks for posting on the Community page, if you're looking at the report that says for example Sales by  Product/Service Detail so reports> scroll down to sales and customers> select Sales by  Product/Service Detail report> the little cog wheel top right of the report next to the export and printer icon> show more>tick net amount> click back on the report and it will now show you a column with the net amount values in.