First, ensure you have your Custom Columns set to show you Gross Revenue and Original Cost/ Purchase Cost/ etc. (Article)
After, select the date range you'd like to report on (Article)
Then, select the inventory you want to report on and download your inventory with Selected Columns (Article)
Next, open the email with the attachment and open it in Google Sheets (can be done in Excel too, but I find Google Sheets easier).
When in Google Sheets, you may want to drag and drop the Gross Revenue Column next to the Original Cost column.
Then add a column to the right and label it 'Return' (or something similar).
In the first row of the RETURN column, add in the function:
=MINUS(<Gross Revenue>,<Original Cost>)
This will subtract the Original Cost from the Gross Revenue to show you how much you earned.
Now let's apply this to all the rows below.
Select the first row of RETURN (with the function)
Hold: Command/Control+Shift+The Down Arrow