<script type="text/javascript">
  function inputMulti() {
      var a = document.getElementById('a').value;
      var b = document.getElementById('b').value;
      document.getElementById('display').innerHTML = a * b;
  }
</script>

<form id="form1" name="form1" method="post" action="">
  <label>multiplikator</label>
  <input type="text" name="a" id="a" onkeyup="inputMulti();" />
  
  <label>Ressourcen</label>
  <input type="text" name="b" id="b" onkeyup="inputMulti();"/>
  
  <div id="display">-</div>
</form>