JSFiddle debugger example

HTML

<html>

  <head>
    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
  </head>

  <body>
    <table>
      <tr>
        <td>Cijena dijela (bez rabata)</td>
        <td><input class="txtBox" type="text" name="dio" value="" /></td>
      </tr>
      <tr>
        <td>Ugradnja (radni sat)</td>
        <td><select class="txtBox" type="textarea" name="sat" value="" />
          <option value="0">0</option>
          <option value="30">0.5</option>
          <option value="60">1</option>
          <option value="90">1.5</option>
          <option value="120">2</option>
          <option value="150">2.5</option>
          <option value="180">3</option>
        </td>
      </tr>
      <tr>
        <td>Marža</td>
        <td><select class="txtBox" type="textarea" name="rabat" value="" />
          <option>0 kn</option>
          <option>50 kn</option>
          <option>100 kn</option>
          <option>150 kn</option>
          <option>200 kn</option>
          <option>250 kn</option>
          <option>300 kn</option>
          <option>350 kn</option>
          <option>400 kn</option>
          <option>450 kn</option>
        </td>
      </tr>
      <tr>
        <td>Cijena bez PDV-a</td>
        <td><input type="text" id="bez" name="bez" value="0" /></td>
      </tr>
      <tr>
        <td>Cijena s PDV-om</td>
        <td><input type="text" id="sa" name="sa" value="0" /></td>
      </tr>
    </table>
    
    <script>
    
      $(document).ready(function() {
        var bez = 0;
        var sa = 0;
        window.setInterval(function() {
          $('.txtBox').each(function() {
            var txtBoxVal = $(this).val();
            bez = bez + Number(txtBoxVal);
            sa = bez * 1.25;
          });
          $('#bez').val(bez);
          $('#sa').val(sa);
          bez = 0;
          sa = 0;
        }, 100);
      });

    </script>
  </body>

</html>