BADI - User Exit for Transaction CJ20N Saving Purchase Price at Activity - Network Level

[DailyJava]

Hello Experts,

Here's the requirement

I need to add mark-up % for every item's purchase price in network's activity and save (Ctrl + S ) the project, that's when the Badi or User-Exit must trigger, but having a hard time to find the right User-Exit or BaDi for this task.

Will you please help me out to do this.
Thank you and regards