Several of the shops at WDW sell them. World of Disney at Downtown Disney also does. The last time I priced them they were running around $55 on up depending on how fancy they get. Accessories are usually under $20.
I got an Alice in Wonderland in the Outlets at Christmas for $9.99 so keep an eye out here. They often have costumes. Got a Woody for £18.99 and the previous year a Cinderella for about $20. At least then you don't feel so bad uyin gthe accessories at full price.
Alison