Originariamente inviato da cavicchiandrea
:master:
document.forms[0].IMPORTO.value= parseFloat(c).toFixed(2).replace('.',',')
<SCRIPT language="JavaScript">

function calcolatotale() {

var a = document.forms[0].numero.options[document.forms[0].numero.selectedIndex].text;
var b = document.forms[0].costo.value;
var c = a * b;

document.forms[0].IMPORTO.value=c;

}

function cancella() {

document.forms[0].IMPORTO.value= parseFloat(c).toFixed(2).replace('.',',');
document.forms[0].numero.selectedIndex=0;

}

</SCRIPT>
<form action="process.php" method="post" name="form1" id="form1">
<table border="1" cellpadding="1" cellspacing="0" bordercolor="#CCCCCC">
<tr>
<td nowrap="nowrap"><table height="7" border="1" align="center" cellpadding="5" cellspacing="1" bgcolor="#ccccff">
<tbody>
<tr>
<td width="150" height="20"><font color="#000080" size="1" face="Verdana" class="titoli">Tour</font></td>
<td width="150" height="20"><font color="#000080" size="1" face="Verdana" class="titoli">Quantity</font></td>
<td width="170" height="20"><font color="#000080" size="1" face="Verdana" class="titoli">Prezzo Euro</font></td>
<td width="170" height="20"><font color="#000080" size="1" face="Verdana" class="titoli">Total Price</font></td>
</tr>
<tr>
<td width="150" height="4" bgcolor="#e0e0e0"><font color="#000080" size="1" face="Verdana" class="offerta">Pisa &amp; Lucca Tour</font></td>
<td width="150" height="4" bgcolor="#e0e0e0"> <select name="numero" onchange="calcolatotale();">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
<option>5</option>
<option>6</option>
</select> </td>
<td width="170" height="4" bgcolor="#e0e0e0"><font color="#000080" size="1" face="Verdana">
<input name="costo" type=text value="75" readonly />
<span class="titoli">€</span></font></td>
<td width="170" height="4" bgcolor="#e0e0e0"><font color="#000080"><font face="Verdana"><font size="1"></font></font></font><font color="#000080" size="1" face="Verdana">
<input name="IMPORTO" type=text value="75,00" readonly id="IMPORTO" />
<span class="titoli"> €</span></font></td>
</tr>
</tbody>
</table> </td>
</tr>
<tr>
<td nowrap="nowrap"></td>
</tr>
</table>
</form>




Ho sostituito la stringa con la tua, ma continua a darmi come totale un numero intero, privo dei due decimali.

Se invece come costo del biglietto invece di 75 gli dò ad esempio 75,00 mi dà come risultato "NaN"...

Dove sbaglio!?