Age and rarity plus condition equal cash. When you are able to mix collecting genres, you also increase desirability. This 1927 "Metropolis" poster hits all the buttons. Mixing art deco styling, the rarity of only 4 known examples, almost perfect condition and 3 bidders with deep pockets made this Fritz Lang classic sci-fi poster sell for an incredible $1.2 million dollars in a recent auction.
If you can't afford the holy grail of posters, you can build a very nice collection for very little. A friend of mine collects horror movie posters and B grade slasher film posters. Many of these are spectacular because of the graphics and subject matter. The can be had for a buck or two apiece or for some of the more popular move posters, $20-$40 each. You can personalize with your man cave with your particular style for a little more than you would pay for ugly wallpaper. If anyone has an original "Wizard of Oz poster" give me a call. It's my holy grail.