Honestly, I don't remember all the details. My daughter probably had around 30 Disney Princess costumes already (my wife bought them after Halloween) but she didn't have a Tiana so she was excited to see it on the shelves in the shop.
Different costumes cost different prices. There were a couple of different options for several princess costumes. There was an expensive option that was more elaborate and a less expensive option.

My daughter is a bit nutty and wears costumes around the house regularly so it wasn't a pain for her to wear her Tiana costume around Disneyland. What made it extra special for her was running into the Tiana character in the park and walking around with her for a few minutes and posing for pictures.
Of course, she also posed for pictures with the other princesses in the area just outside the BBB.

I think we spent a few hundred dollars, but it was definitely worth it. My daughter absolutely loved it.