I couldn't find much about it online but I'm betting it's not cheap. I think I would use a search engine and look up a seamstress in your local area and call and get a ballpark figure. With material and all, I think it could get quite expensive. Hopefully one of the ladies on this site may know more about it and can give you some ideas ! Good luck !
There is no way of estimating that given far to many unknown factors about the pattern to use, the fabric, the hours involved in putting it altogether .. You would have to ask for a quote from one who would know what their abilities are.
You can actually buy the one from the film at the Warner Brothers Harry Potter studio tour gift shop and its only a few hundred pounds, I was honestly so tempted to get it! But to have it made from scratch will be insanely expensive, im guessing upwards of £500 but I really can't say for sure.
I had I little bit taken off the hem of my prom dress and that was £30, so to construct a whole dress and especially one with all those layers is going to be expensive. Im sure you will be able to find something in a similar style and colour if you look around, though. There's so much choice out there you'll definitely find something you like.
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