Event.observe(window, "load", function() {
new Form.Observer('costs_form',0.3, function(form, value){    
	new Ajax.Request(form.readAttribute("action"), {
      parameters: form.serialize(true),
      onLoad: function() {},
      onSuccess: function(transport) {            
		  var json = transport.responseText.evalJSON();	  
		  $('cost').innerHTML = 'USD$'+json.cost.toFixed(2);
	  }
    });
})
});