Amazon has the Disney Princess 2-in-1 Ballgown Surprise Cinderella Doll on sale for $9.99 right now (regular price: $20.99). You can also get the Disney Princess 2-in-1 Ballgown Surprise Tiana Doll for $9.99. These both ship free with Amazon Prime or a $25 Super Saver Shipping order.
If you’ve never seen these, check out this video of how they transform.