sto spegnendo e non posso controllare. L'ho fatto solo per esercizio e senza alcuna pretesa:
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
	<head>
		<title>a1</title>
		<meta content="JavaScript" name="vs_defaultClientScript">
		<meta content="http://schemas.microsoft.com/intellisense/ie5" name="vs_targetSchema">
		<meta content="Microsoft Visual Studio .NET 7.1" name="GENERATOR">
		<meta content="VisualStudio.HTML" name="ProgId">
		<meta content="Microsoft Visual Studio .NET 7.1" name="Originator">
		<script language="javascript" id="clientEventHandlersJS">
<!--

function Button1_onclick() {
	var pa = parseFloat(document.getElementById("pa").value);
	var pv = parseFloat(document.getElementById("pv").value);
	var ps = document.getElementById("sconti");
	var sconto = parseFloat(ps.value);

	var pvs = pv*(1-sconto/100);
	document.getElementById("ps").value = pvs;
}

function pa_onchange() {
	var pa = parseFloat(document.getElementById("pa").value);
	var pv = parseFloat(document.getElementById("pv").value);
	
	var smax = getsmax(pa, pv);
	
	var ps = document.getElementById("sconti");
	var pso = ps.options;
	for (var i = pso.length - 1; i >= 0; i--) ps.remove(i);
	
	for(var i = 0; i <= smax; i++)
	{
		pso[pso.length] = new Option(i, i);
	}
	
	
}

function pv_onchange() {
	pa_onchange();
}

function getsmax(pa, pv)
{
	return 100*(pv - pa)/pv;
}


function sconti_onchange() {
	Button1_onclick();
}

//-->
		</script>
	</head>
	<body MS_POSITIONING="FlowLayout">
		<table>
			<tr>
				<td>Prezzo acquisto</td>
				<td><INPUT language="javascript" id="pa" type="text" onchange="return pa_onchange()" value="0"
						name="pa"></td>
			</tr>
			<tr>
				<td>Prezzo vendita</td>
				<td><INPUT type="text" ID="pv" NAME="pv" language="javascript" onchange="return pv_onchange()"
						value="0"></td>
			</tr>
			<tr>
				<td>sconti</td>
				<td><SELECT ID="sconti" NAME="sconti" language="javascript" onchange="return sconti_onchange()"></SELECT></td>
			</tr>
			<tr>
				<td colspan="2"><INPUT type="button" value="Calcola prezzo vendita scontato" ID="Button1" NAME="Button1"
						language="javascript" onclick="return Button1_onclick()"></td>
			</tr>
			<tr>
				<td>Prezzo vendita scontato</td>
				<td><INPUT type="text" ID="ps" NAME="ps"></td>
			</tr>
		</table>
	</body>
</html>