JSFiddle - React, Tailwind, and code Playground

HTML

<select style="float:left;">
      <option value="5">$5.00 monthly</option>
      <option value="10">$10.00 monthly</option>
      <option value="15">$15.00 monthly</option>
      <option value="20">$20.00 monthly</option>
    </select>

<div id="button" style="width:100px;height:30px;float:left;background:#ccc;line-height:30px;text-align:center;margin:auto 5px;"><a href="http://ordernow/5/">Order</a></div>

JavaScript

$('select').change(function(){
    var val =$(this).val();
    $('#button a').attr('href', function(_,href){
        var reg = /\/\d\/$/;        
        return href.replace(reg, '/'+val+'/');
        
    });    
});