Ciao, la pagina che crei deve essere .php

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento senza titolo</title>
</head>

<body>
<form id="form1" name="form1" method="post" action="">


Prezzo
<input name="prezzo" type="text" id="prezzo" value="5.25" />
(separato da punto coi decimali)
</p>


Quantit&agrave;
<input name="quantita" type="text" id="quantita" value="1" />
<input type="submit" name="Submit" value="Calcola" />
</p>


Totale = <?php echo $_POST['prezzo']*$_POST['quantita'] ?></p>
</form>
</body>
</html>


ciao