I would say DEFINITELY let her wear a costume.
In Oct 03, we had PS for CRT. DD wanted to wear a Belle costume, which she didn't have. I told her that we would buy the costume at WOrld of Disney when we got to WDW. Well, once we got there and realized for the dress and the accessories was going to be $100+ we talked her out of it. I felt sooo... bad, when we got to CRT alot of the little girls were wearing Princess costumes and 1 little girl was wearing the same Belle dress that she had wanted. Poor thing, she just stood there and stared at that little girls dress. She still had a good time but I could tell she was dissappointed.
.