They're quite pricey, around $60 I think! You can buy them in Walmart and I think Target aswell for much less, even the Disney Store in the Florida Mall would probably be cheaper and they often have good sales. Loads of places over here seem to have them aswell, I saw them in Costco last Christmas for about £10 (dress and hair bow) and bought a couple for one of those present appeals
I bought the Tinkerbell dress in xxs for my 1 year old DD a few weeks ago and it was $65 + tax It is lovely though and I did not feel too bad spending that much as I got the birthday fun card.
We were in Walmart a few days later and the had some beautiful dresses at a fraction of the price with accessories too and they looked just as good
if you go to the BBB thread, http://www.disboards.com/showthread.php?t=1893384
there is info on there about purchasing costumes that come from WDW. There is a lady called Mrs B who gets the costumes from the world and sells them on really cheaply. Don't know how she does it, but I got a snow white costume and an Ariel ballgown from her for about $20 each.
They are genuine costumes and are fabulous!!! I've got the Ariel costume stashed away so I can bring it out on our trip and I won't have to spend big bucks on a costume at DTD.