Don't assume your dress has a bustle. Even when the salesperson tells you that, "This is how it will look when you bustle it." and "We'll show you how to bustle it at your last fitting." or when they don't ever ask you if you want one put in.
Little did I know most wedding gowns don't come with bustles put in them-- not sure how they thought I would know. It's not like I buy wedding dresses regularly!
Just ask (before you purchase) so you know for sure and then ask for the fee to put one in so you can budget that.