Custom fields to change with a variation quantity

Hello.

I want to know if its possible with the custom fields, add on the product an extra that changes automatically depending one variation.

For example:

Main price: 100 € . I set up a variation “Number of people to the event”… If you choose in the variation your election it changes cause it is a variable product. I need to also If I choose for example on the variation 105 people, the custom field called “Compostable glass + 2€” to change with that quantity choosen on the variation, ( 105 x 2 ) and then the customer is able to check the check box of the extra or not. THX