JSFiddle - React, Tailwind, and code Playground

HTML

<form action="#" method="post">
    <input name="type" value="" type="checkbox" /><br />
    <input name="service_price" value="" type="text" /><br />
    <input name="modem_price" value="" type="text" /><br />
    <input id="total_price" name="" value="" type="text" />
</form>

JavaScript

function compute() {
    if ( $('input[name=type]:checked').val() != undefined ) {
        var a = parseInt($('input[name=service_price]').val());
        var b = parseInt($('input[name=modem_price]').val());
        var total = a+b;
        $('#total_price').val(a+b);
    }
}

jQuery('form').delegate('input[name=type], input[name=service_price], input[name=modem_price]','change', compute);