It's even more cost effective as you aren't paying for a dress that's finished and someone has to do extra work to make it fit or gem it so that you can walk in it. Pay once for the job to be done right first time. The other advantage is that if you suddenly decide it suits better to change a bit you can do at the right time.
Watch out for shops who let you get carried away the first day you are out trying on and they rush you into a decision. Enjoy every second of planning your Big Day it's part of your memory if it. If you have a bad experience take care to guard yourself as you really want every second of planning your wedding to be perfect. Don't just say 'that will do' you will regret that. Take your time make sure you are making the right choice for You.
I won't touch dresses from other shops but I will help you with accessories such as shoes, foundation garments, headdresses and veils.