You would need to create an Invoicing statistics report, adding in the “Price Item” to the “Sort” field and then adding the price items you wish to report on.  Then within the “Values Used” field on the “Invoicing” tab of the report select “Statistical Weeks”

There is a helpsheet (no.HS240) which covers this.  Please click on the following link to view or download helpsheet – HS240 Statistics – Invoicing – Student Weeks by Price Item.docx