If you're new to Disney Princess costumes, consider this a primer on the many types of costumes you might see for any given character! Below are the types/categories of princess costumes you will find available (at least, as I think of them).
DISNEY STORE COSTUMES
"Official" Disney costumes available at the Disney store. There may be a "Deluxe" version of the costume and a "regular" version of the costume for the same character. The regular versions usually retail for under $50, while the "deluxe" costumes might be closer to $80. Often the two levels of costumes will share trim details so that they can both work with the accessories (shoes, gloves, etc.) that are available at the same time.
LIMITED EDITION COSTUMES
These are beautiful, more expensive costumes, that retail around $150 each. They are often limited to a quantity of between 1,500-3,000. So far LE costumes have been released for: Snow White, Belle, Rapunzel, Rapunzel Wedding Dress, Cinderella, Arial, Anna (Traveling Costume), Elsa (Snowflake Costume), and Minnie Mouse. If you ever see a LE costume on eBay for less that $200, snap it up! I've seen Snow White, Belle, and Rapunzel (by far the rarest of the group) sell for upwards of $500 consistently, and of course you've heard about the Frozen dresses going for over $1,000 right now. It is the LE versions of the costumes that the articles are talking about. By being persistent I was able to find the Belle and Rapunzel gowns on eBay at just a smidgeon above what they cost new. The rest in the girls' collection we bought new using coupons and discounts.
THEME PARK COSTUMES
This is just my observation, but it seems to me that the costumes available in the park have been designed for length and weight. That is to say, they are generally shorter than those available from the Disney Store and lack embellishment, I think to keep them as light as possible in the Florida heat. You can buy costumes at a few places throughout the parks, specifically inside the World of Disney Store in Downtown Disney and across from Cinderella's Castle at Castle Couture in Magic Kingdom. You can also choose to buy a costume as part of the more expensive Bibbidy Bobbidy Boutique packages. I will need to compose a post about that in more detail later, but suffice to say I don't recommend buying the theme park costumes. There's no requirement that you buy a costume in the park, so your child can arrive in a princess costume from the Disney Store purchased before hand which (in my opinion only) tends to be prettier and significantly less expensive. I recall the in-park costumes averaging between $60-80 and, as I described, are not as embellished as the Disney Store versions.
Other retailers such as Toys R Us, Walmart, Target, halloween shops, and department stores like Macy's and Nordstrom's will have princess costumes, too. Especially around the Halloween season. I've met lots of parents at Disney World who were able to get great deals on fantastic quality costumes by waiting until right after Halloween and snapping them up on discount. All of Eva's small size princess costumes are from Babies R Us and I think they're beautiful quality and very affordable (I think they were under $20 each). With that exception, I don't buy many princess costumes from non-Disney retailers so I can't faithfully review them except to say they're out there!
In recent years many folks have turned to Etsy for unique Princess-inspired costumes and dresses. Some seek to be faithful recreations of what's available in the stores or in the movies, while others are inventive interpretations. Some create dresses that Disney has overlooked in its merchandising or are hopelessly out-of-production like Giselle's day dress, or Elsa Coronation grown. Generally speaking, the Etsy prices rival the Disney Store prices, so I'm not sure that this is an avenue if you're looking to save money so much as if you're looking for something unique and different, that sets your child apart from the throng of Princesses. I'm constantly amazed but the creativity and capability of the many talented craftspersons on Etsy!
EBAY RESALE COSTUMES
I have purchased most of our collection of princess dresses off of eBay over the course of several years. Because kids grow so fast, a lot of the dresses hold up well enough before they outgrow them to be resellable. If you're shopping on eBay my advice is this: understand the style of the dress you are buying (is it a Limited Edition, Deluxe, etc.). A lot of times eBay sellers will claim a dress is LE or Deluxe and try to charge a higher price when it is not. Make sure you look carefully at the pictures for any damage. And most importantly, BE PATIENT. Set up a saved search and wait... eventually, the dress you want will come up at the price you want if you just wait. The best time to buy princess dresses from eBay is right before and after halloween when the site is simply flooded with costumes.
I've known a lot of parents who buy the Disney Store Princess Nightgowns for their kids as dress up costumes. They're really lovely gowns with embroidered princess cameos. We discovered them a few years back because my younger daughter Eva was fanatic about sleeping in nightgowns instead of pants sets (the exact opposite of her older sister, I might add!) Because these nightgowns retail from the Disney Store for about $29, we make due with those we can find either on eBay or on clearance with a coupon. I try not to pay more that $10 for the pajamas because at this age Eva just grows out of them too quickly. As I said, I've known lots of folks who buy these as dress up dresses and they're certainly fancy enough for that! In fact, I'm hoping to see the Elsa nightgown restock soon so I can buy one for Eva for that purpose, since it seems like a waste to buy the expensive Elsa costumes in size 3!
One last tip: The Disney Store costumes are designed shorter so that the skirts end mid-calf for safety reasons. I've always bought costumes a few sizes too big because Viola likes them to touch the floor like "real ball gowns". WHILE THE DRESSES ARE TOO LONG, WE SIMPLY PUT ONE OR TWO TUTU'S UNDERNEATH!, and as Viola grows into them we put less tutu's or no tutu's underneath. This has allowed us to get years of use out of size 5/6 costumes, which Viola has been wearing since she was about 4.
My observation (which, to be honest, only goes back so far as Tangled) is that when a new princess movie comes out you can expect to see the princess' costume hit the Disney store about two months in advance. It's my opinion that these are the nicest quality and most movie-accurate versions of that princesses costume that you will find. From that point on, many iterations of the costume will become available both through the Disney Store and other retailers such as Toys R Us, Target, etc. each one less faithful to the movie. My opinion is that the best time to buy is before the movie, or around Halloween.