Yeah, I'm finding it hard to think of an answer to his question, because there really is no right answer. Star Wars posters came to mind for me right away, but I only thought of that because I personally don't like the movies. It's no shock that they go for as much as they do, because they have a large die-hard following. Essentially these fans are the ones that have set these high prices themselves, so I can't really say it's overpriced. Don't know if that made any sense, I need some sleep
P.S: Semi-related but I just met my first real life fellow poster collector last week, who of course collects Star Wars posters and related memorabilia. I just about passed out when he told me he had around 50k invested into his Star Wars collection