I've been looking into this as well. I've found several reports the dresses in the Bibbidy Bobbidy Boutique which are provided if you book the package that includes a costume are around $65. Then I watched videos of various family's Boutique experiences and saw the kids picking out dresses I see on the Disney shop website for around $45. It took me a while to figure out what was up but they don't just get the dress. If you read the description on the Disney World website and look closely in the videos it is "dress and accessories". They receive the costume, crown, and sceptor which brings the total to around $65.
There is also a version that is more like a real pageant dress. These gowns retail on the Disney website for $350 each.
You definitely can get the same costumes elsewhere for less. Ebay, Amazon, and even brick and mortar retailers like Target will sometimes have the same costumes. Reports in all the blogs and forums I found regarding just buying the cheaper hair, makeup, and nails package and bringing your own costume from home were that it is very common and they don't at all shame folks for it. I found some videos of people bringing in their own or even bringing in their kids already wearing princess costumes and everyone on staff was very positive about it. In one video they even used a crown the little girl brought in with her.
I'm actually thinking I will make my granddaughter's dresses for their Bibbidy Bobbidy Boutique experience. I've seen several reports the kids find the costumes get itchy in the heat and to take a change of clothing for later in the day. I was thinking I would make them t-shirt dress versions of their favorite princess costumes. I found a tutorial where they used puff sleeved little girl's t-shirts to make Belle, Cinderella, Aurora, Snow White, and Little Mermaid versions with ribbon-trimmed tutu skirts and shorts. They should be much more comfortable to wear.