You would build this formula in our C2C section
To create a formula for revenues or expenses where you are multiplying Price x Quantity, you will need to build a C2C type formula.
First, you'll need to create a non-financial group (go to Settings > Manage Groups and select Add Group, make sure to choose NON-FINANCIAL as your mapping Identifier)
Now, create the categories to host the Price and Quantity data, and add them to the Group you just created - go to Settings > Manage Categories, select Add New, and fill in the form, choosing your new group from the first menu.
Finally, go to Data/Formulas > Manage C2Cs, hit Add New, and fill in the form, choosing your target revenue or expense account as your target category, and your Price and Quantity categories as your sources.
Now you have a formula. To audit this formula, you can click on the Target Category name in the Manage C2Cs table (it's blue, so it's a link). This will take you to the Data Sheet for that account, and if you click the Input icon (next to the fx on the sub category row) you'll be able to see the inputs. Click the Edit tool (the pencil icon) before clicking the Input icon, and you'll be able to edit the inputs from the pop up.
All non-financial accounts are edited in exactly the same way as your financials, use the COA menu at the top left of the Data Sheet to search.
Watch this video to learn more about formulas: