codice:
<form id="form1" name="form1" method="post" action="" 
onsubmit="return calcola();">
<label>
<input name="quantita" id="q1" type="text" value="5" /> *   
</label>
<label> 
<input name="cifra_fissa" id="q2" type="text" value="50.00" /> +    
</label>
<label>
<input name="cifra_da_sommare" id="q3" type="text" value="84.00" />
</label>
<label>
<input type="text" name="totale" id="totale" value="?" />
</label>


<input type="submit" value="calcola" />


</form>


<script type="text/javascript">
  function calcola() {
     var q1 = parseFloat(document.getElementById('q1').value);
     var q2 = parseFloat(document.getElementById('q2').value);
     var q3 = parseFloat(document.getElementById('q3').value);

     document.getElementById('totale').value = (q1 * q2) + q3;
     return false;
  }
</script>