codice:
<head>
<script type="text/javascript">
function setValue(selectValue){
  var newValue='';
   switch(selectValue){
      case '27': newValue=4075; break;
      case '28': newValue=4175; break;
      case '29': newValue=4275; break;
      //ecc.
   }
   document.getElementById('targetInput').value=newValue;
}
</script>
</head>
<body>
<select onchange="setValue(this.value)">
    <option value="27">27</option>
    <option value="28">29</option>
    <option value="28">29</option>
    
</select>
<input type="hidden" name="inputName" id="targetInput" />
</body>