Lisa,
Unique but pricey sums them up perfectly! The dresses sold in Disney World are WDW exclusives so you won't find them anywhere else. But there is a price to pay for that exclusivity. Namely, around $65-$90 depending on which character you choose. An of course that's only for the dress - the adorable matching shoes, crowns, wands, and other trinkets are all sold separately.
I suggest visiting the
www.disboards.com forum. Look in the 'Trip planning' section, then under 'theme parks attractions and strategies' and the thread titled '
Bibbidi Bobbidi Boutique ~ ALL you need to know + Pix of NEW costumes!' (You can search their forum for this thread also) While they are talking about the BBB, there are also incredibly detailed pictures and prices for every costume available, current as of 7/22/08. This should help you determine what your expense would be.
For what it's worth, we bring our own costumes from home! For one daughter we might splurge, but for 3 we bring them along!
Hope that helps!