Carebear - I found that getting my dress made by a dressmaker was far cheaper than any I had tried on. But that was not the only factor.
Nothing I had tried on fitted just perfectly - bridal shops can order longer hemlines that's about it - if you're taller/shorter than average the options start getting limited.
But a cheaper dressmaker-made dress can also look a bit homemade - you definitely get what you pay for by choosing a good bridal dressmaker and good fabrics.