That dress was still very extremely expensive though, and said to be designed by one of the best fashion designers to make Emma Watson as beautiful as possible, and perhaps the fashion designers wanted to put more emphasis on making Emma the most beautiful girl in the world than on following the book to the letter.
The dress did kinda scare Emma though, and she didn't dare to put it on, especially not prior to when she had to for the shooting, as she was very extremely afraid she'd damage it.
Personally I think the dress creators were right... It was more important that Hermione was so beautiful that none of her fellow students wanted to believe it was her (not to mention that the fact she looked that beautiful while dating Viktor Krum pissed off Ron even more) and that even the girls were in awe over Hermione's feminine beauty. And to be honest, judging as a heterosexual man, that was what the dress did. Emma alias Hermione *did* look more beautiful than I could ever imagine her to look (and then to think she was never bad looking from the start). Now you could say I am judging too much as a man, but remember, she *was* on (sort of) a date.
Also note that sometimes actors themselves also have a say in stuff like this, but if Emma actually made a preference notice to the dress designers or not is something I don't know.