la funzione è questa:
</script>
<script type="text/javascript" language="JavaScript">
<!--
function updateSum( __hForm, __productEl, __quantityEl, __priceEl )
{
var _hProduct = __hForm.elements[__productEl];
var _hQuantity = __hForm.elements[__quantityEl];
var _hPrice = __hForm.elements[__priceEl];
if ( _hProduct.selectedIndex == -1 )
_hPrice.value = "0 €";
else {
_hPrice.value = parseFloat( _hProduct.options[_hProduct.selectedIndex].value ) *
parseInt( _hQuantity.value ) + " €";
}
} //-->
</script>
