devote,
 убедил 
 
<input id="test_value">
<input type="submit" id="test_submit" disabled="true">
<script>
  window.onload = function () {
    var inp = document.getElementById("test_value");
    var sub = document.getElementById("test_submit");
    inp.oninput = function () {
        sub.disabled = this.value < 100;
    }
  }
</script>