Una cosa del genere
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN">
<html>
<head>
<title>test</title>
<script type="text/javascript">
function Somma(valore){
var importo = document.getElementById("Valore2").value;
var int = valore.value
var Tot = (int*importo);
document.getElementById("Totale").value=Tot;
}
</script>
</head>
<body>
<form id="form1" method="post" action="">
Scrivi l'importo <input name="Valore1" type="text" id="Valore1" onkeyup="Somma(this)" /> x <input name="Valore2" id="Valore2" type="text" value="10" readonly="true" /> = <input name="Totale" type="text" id="Totale" readonly="true" />
</form>
</body>
</html>