JSFiddle - React, Tailwind, and code Playground

HTML

<select id="combo">
    <option value="1">Test 1</option>
    <option value="2">Test 2</option>
    <option value="3">Test 3</option>
    <option value="6">Test 6</option>
    <option value="4">Test 4</option>
    <option value="5">Test 5</option>
</select>


<select id="combo1">
    <option value="1">Test 1</option>
    <option value="2">Test 2</option>
    <option value="3">Test 3</option>
    <option value="6">Test 6</option>
    <option value="4">Test 4</option>
    <option value="5">Test 5</option>
</select>

<br>
    <input type = "text" id = "firstvalue" value= "Test">

JavaScript

$(document).ready(function() {
    price1=0;
    price2 =0;
    $('#combo').change(function() {
          price1= this.value;
          $('#firstvalue').val(parseInt(price1)+parseInt(price2));

    });
    
    
    $('#combo1').change(function() {
        price2 = this.value;
        $('#firstvalue').val(parseInt(price1)+parseInt(price2));
    });
});