<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 () {
      if (this.value > 100) {
        sub.disabled = false;
      }
      else {
        sub.disabled = true;
      }
    }
  }
</script>

<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>