How do I assign total price depending on the choice they make in the form field?

How do I assign total price depending on the choice they make in the form field?

Do you want to perform a calculation inside of the form and pass it to the payment field? If so, please check the following guides:

How to Perform Form Calculation Using a Widget

How to Pass a Calculation to a Payment Field

If I did not understand your question correctly, please explain in more detail what exactly do you want to achieve and provide us a link of the form you are working on.