From the course on the Enrolments & Results area, select the clients and navigate to the RAPT tab
As highlighted in the screenshot below, enter the cap value:

Please note setting this field will not effect RAPT reporting
However, it will determine how the invoice lines (one for each Unit of Competency) are created (using the Unit RAPT Fees menu item) they will be capped to a value of $400 in this case see the below screenshot for an example: