They are expensive because of the status factor. They are "cool". It is now cool to say that your pics happened by accident and luck, and have enough weight to be good based on some technical concern such as your use of a plastic camera. Some day soon, the opposite will be back en vogue.
If I happen to get a great shot with a Holga, Diana, or any other camera, I guess I have the option of proclaiming it, and maybe being cool. People will think I am so badddd ass because I shun technique.....
WHATEVER - I see it as a usable photographic tool in the art of printmaking. When you want something with a certain mood or look, there is nothing like them. $40 seems relatively cheap for the Diana, especially with the new features and the book that comes with it. The Holga at $35 seems cheap too. As long as you view them as a means to an end, and not simply as a CONSUMER PRODUCT judged on surface value alone, they are well worth the money, IMO.
The second someone mentions gear in an artist's statement, a red flag goes up for me, whether it be an overly technical obsession with gear, or a heavyhanded statement about the wonders of shooting without technique.
The only time plastic camera images work, IMO, is when the subject matter somehow relates conceptually to the fact that it is a "cheap toy"; when the camera is used to talk about or create pix making cultural commentary on ways in which a person might have used the camera in all seriousness before it became a cult icon. IMO, very good for a cheap suburban nostalgic mood.