$(function(){
  $("select option:first-child").attr("selected",true);

  $("[id^='product_amount']").change(function(){
    var $sum = (Math.round(parseFloat($(".price",$(this).parents(".product_price")).text() )*100)* parseFloat($(this).val()))/100;
    $("span.total_amount_product",$(this).parents(".product_price")).text($sum);
    $("[id^='product_price']",$(this).parents(".product_price")).val($sum);
    var $total_sum = 0;
    $.each($("span.total_amount_product"),function(){
      $total_sum = $total_sum+(Math.round(parseFloat($(this).text())*100));
    });
    $("span.total_amount").text(parseFloat($total_sum)/parseFloat(100));
  });
});