JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<div id="suma">
  <div>
    <label>Cant.</label>
    <input type="text" name="cant[0]" data-id="0">

    <label>Precio Unitario</label>
    <input type="text" name="price[0]" data-id="0">
 
    <label>Monto</label>
    <input type="text" name="amount[0]" data-id="0" disabled>
  </div>
  
  <div>
    <label>Cant.</label>
    <input type="text" name="cant[1]" data-id="1">
    *
    <label>Precio</label>
    <input type="text" name="price[1]" data-id="1">
     = 
    <label>Total</label>
    <input type="text" name="amount[1]" data-id="1" disabled>
  </div>
  
  <div>
    <label>Cant.</label>
    <input type="text" name="cant[2]" data-id="2">
    *
    <label>Precio</label>
    <input type="text" name="price[2]" data-id="2">
     = 
    <label>Total</label>
    <input type="text" name="amount[2]" data-id="2" disabled>
  </div>
  
  <div>
    <label>SUBTOTAL:</label>
    <input type="text" name="subtotal" id="subtotal" disabled>
  </div>
  <div>
    <label>IVA 10%</label>
    <input type="checkbox" name="iva" data-iva="10">
    <input type="text" name="ivaAmount" id="iva" disabled>
  </div>
  <div>
    <label>TOTAL:</label>
    <input type="text" name="total" id="total" disabled>
  </div>
</div>

JavaScript

$(document).ready(function(){
  $('#iva').hide();
  $('#suma input').blur(function(){
  	var id = $(this).data('id');
    costs(id);
  });
});

function costs(id){
  var cant = $('input[name="cant['+id+']"]').val();
  var price = $('input[name="price['+id+']"]').val();
  var cost = cant * price;
  $('input[name="amount['+id+']"]').val(cost);
  
  theTotal();
}

function theTotal(){
	var subtotal = 0;
  var ivaChecked = $('input[name=iva]').is(':checked');
  var iva = $('input[name=iva]').data('iva');
  
  $('input[name^=amount]').each(function(){
    subtotal += Number($(this).val());
  });
  
  $('#subtotal').val(subtotal);
  
  if(ivaChecked){
    iva = subtotal * (iva/100);
    subtotal = subtotal + iva;
    $('#iva').val(iva).show();
  }
  
  $('#total').val(subtotal);
}