It's great that someone is documenting this module. Having worked on this module for a while, it's hard to see which parts are confusing to users and need documenting the most
Now to answer your questions:
1- The discount code is a free text field. Currently, the Face-to-face module doesn't verify that it is correct, that's up to the teacher to do. So basically, if you write anything in the discount code field, then you automatically get the discounted price.
2- The cost column is not actually in the gradebook, but rather in the attendance list that can be exported as a spreadsheet. This export is done through the screen that shows all available/past sessions.
3- The gradebook interaction is currently limited to submitting a grade of 0 if the student has not attended and a grade of 100 if the student has attended.
Cheers,
Francois