How can i sum this up? (with generated disabled input fields)

Hello i am trying to add up two input fields that are not inserted with keyboard. But are just disabled input fields. If i choose dates the input field will generate a price function setFullPrice(), and the other input field gets generated from checking a checkbox with a price if($('#electricityCheckbox').is(':checked')). But how do i sum up the 2 input fields and output in #totalPrice.

I tried to do it with code online but i saw only explanations to sum up fields that you fill in, that's not the case with me.

    function setFullPrice() {
      if(getDays() > 0) {
    $('#overNightRate').val(getDays() * 200);
      }
    }

if($('#electricityCheckbox').is(':checked')) {
       $('#extraPrice').val(10 + ' euro');
     }

<input id='overNightRate' type='text' name='overNightRate' class='form-control addUpFields' placeholder="Bedrag word berekent" disabled/>

<input id='extraPrice' type='text' name='extraPrice' class='form-control addUpFields' placeholder="Bedrag word berekent" disabled/>

<input id='totalPrice' type='text' name='totalPrice' class='form-control' placeholder="Bedrag word berekent" disabled/>

I appreciate your time, thanks (i am beginner in Jquery sorry for if this is a stupid question).