<script type="text/javascript">
function UpdateCost() {
  var sum = 15;
  var gn, elem;
  for (i=0; i<5; i++) {
    gn = 'add'+i;
    elem = document.getElementById(gn);
    if (elem.checked == true) { sum += Number(elem.value); }
  }
  document.getElementById('totalcost').value = sum.toFixed(2);
}
</script>


<form action="dodajwszystko.php" enctype="multipart/form-data" method="post">
<input type="checkbox" name='pierwszy' id='add0' value='10'  onclick="UpdateCost()">10<br>
<input type="checkbox" name='drugi' id='add1' value='20' onclick="UpdateCost()">20<br>
<input type="checkbox" name='trzeci' id='add2' value='30' onclick="UpdateCost()">30<br>
<input type="checkbox" name='czwarty' id='add3' value='45' onclick="UpdateCost()">45<br>
<input type="checkbox" name='piaty' id='add4' value='80' onclick="UpdateCost()">80<br>
<input type="text" id="totalcost" name="totalcost" value="15">

<input name="submit" type="submit" value="podlicz">
</form>