<form action="" method="POST">
    <input type="text" name="text1" id="text1" class="sum" /><br />
    <input type="text" name="text2" id="text2" class="sum" /> <br />
    <input type="text" name="text3" id="text3" class="sum" /> <br />

    <input type="checkbox" id="chk1" value="4" class="sum" /><label for="chk1">4</label> <br />
    <input type="checkbox" id="chk2" value="10" class="sum" /><label for="chk2">10</label> <br />
    <select name="u1"  class="chkOptions">
            <option value="0" selected="selected">нет</option>
    <option value="120">1 упаковка</option>
    <option value="240">2 упаковки</option>
    <option value="300">3 упаковки</option>
</select>
    <input type="text" name="text" id="text">
</form>
<script>
$(function() {
    var calc = function() {
          var sum = 0;
          fields.each(function() {
              var el = $(this),
                  type = el.attr("type");

              if ((type == "text") || (type == "checkbox" && el.attr("checked"))) {
                  sum += parseInt($(this).val(), 10) || 0;
              }
              $('#text').val(sum);

          });//end each
        },//end calc

        fields = $(".sum");

    fields.change(calc);

});
</script>