I will add this thought...
I have done or helped with costumes for our community theatre, both the Young Footliters and for some of the adult shows. If you already have/are given/ or buy a beautiful princess-type dress you may just want to add a crown, wand or sceptre, jewelry and/or shoes to complete the outfit.
You can find some of the most amazing shoes at second hand stores (small adult sizes fit little girls too!) for bargain prices if you are so inclined.
A plain basic princess crown (from DollarStore or similar) is so much fun for your child to decorate herself with a bag of inexpensive glue-on jewels (save some for the sceptre and shoes)...here's another idea, use her own shoes that she never wears for some reason or other and decorate them to get some use out of them before she outgrows them.
A sceptre or wand can be made out of a dowel (or cut a common plastic hanger a breadknife if you don't have a saw (notch where you want to cut it with the breadknife and it will break when you bend it (freezing it for a few minutes first will make this a snap *grin*) Glue or hot glue a decorated cardboard or posterboard (I use the white inserts from nylon or shirt packages) star on the top...add ribbons (hair ribbon, paper ribbon, gift box ribbons, whatever you have) under the star.
lol...if you like this kind of thing it's easy and the kids love it...if you hate doing this just buy the costume from e-bay and the kids will love that too.