[Forminator Pro] Event registration form : add custom field as price unit

Hi there!
I’m quite stuck with my situation. Here is my demo page : dev.collectifweb.ca/secondsouffle/formation/shine-my-life-formation-grand-public/

I created some forms for people to register on an event.
So basically there is a :
QTY PRICE (number of participant)
UNIT PRICE (custom field on the post)
TOTAL (calculation field)

I’d like to know how to generate a UNIT PRICE field, so basically autopopulate a number field with the event price.
The event price is set as a custom field named prix.
Can you help me on this?

And also, how can I “lock” this field so people can’t change it?
I’m ok to change this field as a hidden field but as I saw, we can’t use calculation on hidden field.

Thanks