In my experience, cakes are always more expensive when you name a specific ride or movie. The 6” cake you were quoted would likely have the wallpaper design, maybe a 2D image of the hitchhiking ghosts, etc. If you want something cheaper, ask for a more abstract design. For example, maybe a deep purple cake with black roses? If you do the work to think of something HM-inspired yourself, you can provide a bit of direction that will likely bring the price down.
One point to note: they can’t fit the wallpaper design on a mini cake. Or at least they haven’t been able to for the years I’ve read this thread.
PS: sorry, Lurkyloo, for answering so many questions. I’m channeling my excitement about an upcoming trip! hah