<html>
<head>
<script type="text/javascript" language="javascript">
var current = 1, money = 30;
function check() {
var select = document.getElementById('select');
if (parseInt(select.value) > money)
select.selectedIndex = current;
else
current = select.selectedIndex;
}
</script>
<style>
</style>
</head>
<body>
У вас на счету 30р<br />
<select id="select" onchange="check();">
<option value="10">10р</option>
<option value="20">20р</option>
<option value="30">30р</option>
<option value="40">40р</option>
<option value="50">50р</option>
</select>
</body>
</html>