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>