Ho provato come mi hai indicato ma nulla.
codice:
<script language="javascript" type="text/javascript">
         window.onkeypress = setInterval('changeTotale()',100);
         function changeTotale(){  
         var somma = <%=scontato%>; 
         var listSelect = document.getElementsByName("supplemento");
         
         for(var i=0; i<listSelect.length; i++){
         curSelect = listSelect[i]; 

     selNum = curSelect.options[curSelect.selectedIndex].value;
     selNum = selNum.replace(/,/g, '.');
     somma += Number(selNum);

         var importol = somma.toFixed(2);    
         var importos = importol.replace('.',','); 
           }             
         document.getElementById('risultato').innerHTML = "€ " +formatCurrency(importol);
         document.getElementById('risultatob').value = somma;
         }
         function formatCurrency(number){
         var numberStr = parseFloat(number).toFixed(2).toString();
         var numFormatDec = numberStr.slice(-2); 
         numberStr = numberStr.substring(0, numberStr.length-3); 
         var numFormat = new Array;
         while (numberStr.length > 3) {
         numFormat.unshift(numberStr.slice(-3));
         numberStr = numberStr.substring(0, numberStr.length-3);
         }
         numFormat.unshift(numberStr);
         return numFormat.join('.')+','+numFormatDec; 
}
</script>

<div style="text-align:center;font-size:30px" id="risultato">
Grazie
G.