You must be absolutely sure of what you want, so that dressmaker can give you an accurate quote - which means you have to try on as many dresses as possible - make a note of:
Material - eg - silk satin, chiffon, organza, duchesse satin

The shape - eg empire line or strapless etc, etc
How many layers of fabric? - is there an underskirt?
Details such as embroidary, type of train or whether it requires a bodice? Bodice work will push up the price - but all ready-to-wear wedding dresses that have bodices are …