Well...
Sometimes it's better to buy the expensive cosplays than cheap. Usually, the cheaper ones are damaged upon arrival, poorly made, or fall apart.
Yeah, it sucks a lot of them are overpriced. But, it's a price well paid (haha) to ensure the product is of good quality. Of course, you can always make your own with modified goodwill clothes and such.
But just remember, it's most likely that most money made off of their cosplays go in for the materials and shipping. I'd find a few Harry potter cosplay blogs and see where they bought their cosplays, or see how much it cost them to make theirs.
Sorry I couldn't do anything but suggest some search tips...
Hope this helped, and have fun with your cosplay!!