[bes]

<table id="myTable">
<tr>
  <th><input type="checkbox">
  <th><input>
</tr>
<tr>
  <th><input type="checkbox">
  <th><input>
</tr>
</table>
<button id="myButton">sum = 0</button>

<script>
window.onload = function () {
  var myTable = document.getElementById('myTable');
  var myButton = document.getElementById('myButton');
  var rows = myTable.rows;
  var len = rows.length;
  var checkbox;
  var inp;
  var sum;


  myButton.onclick = function () {
    sum = 0;
    for (var i = 0; i < len; i++) {
      checkbox = rows[i].children[0].children[0];
      inp = rows[i].children[1].children[0]; 
      if (checkbox.checked == true && isNaN(parseFloat(inp.value)) != true) {
        sum += parseFloat(inp.value); 
      }       
    }
    myButton.innerHTML = 'sum = ' + sum;
  }

}
</script>