How to change input values ​​when opening a modal window?

Question

css

https://midavr.github.io/
When you click on the buy button, a modal window opens and in the select you can select the desired item while changing the input values, but I can’t figure out how to immediately make changes to the inputs in accordance with the selected value in the select.
The value of the select when opening is assigned like this

$(document).ready(function(){
  $('a[data-toggle="modal"]').click(function(e){
  e.preventDefault();
  $("#field304904").val($(this).attr('data-sel'));});
});

And this changes the values ​​of the inputs

function myFunction(e) {
            document.getElementById("id_product3").value = e.target.value



            if (document.getElementById("id_product3").value == 'prod 1'){
                document.getElementById("id_price3").value = '299';
            } else if (document.getElementById("id_product3").value == 'prod 2'){
                document.getElementById("id_price3").value = '299';
            } else if (document.getElementById("id_product3").value == 'prod 3'){
                document.getElementById("id_price3").value = '299';
            } else if (document.getElementById("id_product3").value == 'prod 8'){
                document.getElementById("id_price3").value = '389';
            } else if (document.getElementById("id_product3").value == 'prod 4'){
                document.getElementById("id_price3").value = '449';
            } else if (document.getElementById("id_product3").value == 'prod 5'){
                document.getElementById("id_price3").value = '459';
            } else if (document.getElementById("id_product3").value == 'prod 6'){
                document.getElementById("id_price3").value = '479';
            } else if (document.getElementById("id_product3").value == 'prod 7') {
                document.getElementById("id_price3").value = '489';
            }
        }

Answer 1

It's best to use a pattern and not write 100500 else if, or at least loop through and make a match between cards and options in the select.
But, if you started writing like this, then the least pain will be if you do it like this:
Take the data-sel value and put it in the desired input;
Extract a numeric value from price_old/price_new and place it in the required input.