Scusami fcaldera,

ma con lo script che mi hai dato unito a quello mio non funziona:

codice:
<script language="JavaScript" type="text/javascript">
function UpdateCost() {
  var sum = 0;
  var gn, elem;
  for (i=1; i<7; i++) {
    gn = 'lingue_'+i;
    elem = document.getElementById(gn);
    if (elem.checked == true) { sum += Number(elem.value); }
  }
  document.getElementById('q1').value = sum.toFixed(0);
} 
</script>
<script type="text/javascript">
  window.onload = function() {
      var Fq1 = document.getElementById('q1');
      var Fq2 = document.getElementById('q2');
      var Fq3 = document.getElementById('q3'); 

       Fq1.onchange = Fq2.onchange = Fq3.onchange =
       Fq1.onblur = Fq2.onblur = Fq3.onblur = calcola;


      function calcola() {
         var q1 = parseFloat(Fq1.value) || 0;
         var q2 = parseFloat(Fq2.value) || 0;
         var q3 = parseFloat(Fq3.value) || 0;

         document.getElementById('totale').value = (q1 * q2) + q3;
      }
  }

</script>

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

<input name="lingue_1" id="lingue_1" type="checkbox" value="1" onclick="UpdateCost()" /> 


<input name="lingue_2" id="lingue_2" type="checkbox" value="1" onclick="UpdateCost()" /> 


<input name="lingue_3" id="lingue_2" type="checkbox" value="1" onclick="UpdateCost()" />


<input name="lingue_4" id="lingue_4" type="checkbox" value="1" onclick="UpdateCost()" />


<input name="lingue_5" id="lingue_5" type="checkbox" value="1" onclick="UpdateCost()" />


<input name="lingue_6" id="lingue_6" type="checkbox" value="1" onclick="UpdateCost()" />


<label>
<input name="q1" id="q1" type="text" value="0" /> *
</label>
<label>
<input name="q2" id="q2" type="text" value="50.00" /> +
</label>
<label>
<input name="q3" id="q3" type="text" value="84.00" />
</label>
<label>
<input type="text" name="totale" id="totale" value="?" />
</label>
</form>

ho messo onchange ma non funziona.