<html>
<head>
<title></title>
<script language=javascript>
function calcular()
{
var total;
total = eval(frm.espacio.options[frm.espacio.selectedIndex].value) + eval(frm.ftp.options[frm.ftp.selectedIndex].value) + eval(frm.correo.options[frm.correo.selectedIndex].value);
frm.total.value = total;
}
</script>
</head>
<body onload="calcular()">
<form name=frm >
<select name=espacio onchange="calcular()">
<option value=5>20 Mb</option>
<option value=10>50 Mb</option>
<option value=15>100 Mb</option>
<option value=20>200 Mb</option>
<option value=30>5000 Mb</option>
</select>
<select name=ftp onchange="calcular()">
<option value=2>1 Cuenta</option>
<option value=5>5 Cuenta</option>
<option value=8>10 Cuenta</option>
</select>
<select name=correo onchange="calcular()">
<option value=10>5 Email</option>
<option value=20>15 Email</option>
<option value=40>50 Email</option>
</select>
<input type=text name=total size="8">
</form>
</body>
</html>
Espero que te sirva
