Not too much. The classics always go up. With posters from the 40's and 50's there were only 500 or less made sometimes because they didn't have the number of theatres and goodleplexes we have today. Most posters were returned, beat to hell or thrown away. That's why a decent quality half sheet from the Mummy or Frankenstein can sell for hundreds of thousands of dollars.
With many of the posters there may be only be 50 or a hundred excellent examples left and thousands of people who want them. The prices really skyrocketed when the Japanese realized the classic American posters were WAY undervalued. So a Forbidden Planet one sheet that sold for $200 jumped to $3,000. Again, I'm only talking about the classic stuff. It's like in pinball collecting AFM, MB, TAF, MM, SS, etc, as opposed to Pinbot, which chances are will never be a $6k pin.