Предложу пока такой вариант...
<!DOCTYPE html>
<html>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=windows-1251' />
<script src='https://code.jquery.com/jquery-latest.js'></script>
<!--
<script src="https://code.angularjs.org/1.3.9/angular.min.js"></script>
<script src="https://code.angularjs.org/1.3.9/angular-route.js"></script>
-->
<style type='text/css'>
</style>
<script type='text/javascript'>
$(function(){
$('#ok').click(function(){
var val=$('#new_type').val();
var url='https://btc-alpha.com/api/v1/orderbook/BTC_USD/';
url=url.replace(/[^\/]+(?=\/$)/,val);
alert(url);
});
});
</script>
</head>
<body>
<p>Введите пару торгову : <input type="text" id="new_type" value="ETH_USD"></p>
<button id='ok'>Ok</button>
</body>
</html>