But above all, it is the rule of supply and demand that defines the price.
For example, we know that original movie posters are the most wanted documents on movie collectibles market. Among them, the posters related to cult movies from the Golden Age are always the most coveted, especially the Universal first horror movies. For example the poster of the Mummy (1932) was sold in 1997 near 450.000 $ at Sotheby’s, that made it the top expensive poster for 8 years until 2005, before being dethroned by Metropolis which was sold 690 000 $. Since that, Metropolis price has risen again in 2008 to reach almost 800.000 $ at Drouot.
Sci-Fi movies from the 1950’s are also coveted, as some movies from the 60’s.
Poster Prices are decreasing drastically since the 70’s, especially because of wider print run. Indeed, until 1960’s, a poster print-run was about 3000 to 5000 prints. Since the 1970’s the average print run has jumped to 15.000 to 30.000 prints! The craze for collecting movie art has also started in the 1970’s; until then, movie posters were commonly destroyed after being displayed in movie theatres.
As for movies from the 1980’s, only few posters are exceeding 2000$, which is honorable. Nevertheless, their value is increasing, and they can be considered as an affordable investment.
Most of recent movie posters should not reach such prices. The visual standardization (see chapter “Towards movie poster extinction ?”) and wide print rune, should maintain their prices relatively low. Including the fact that they are sometimes sold directly to the public.
However, some posters from the 2000’s, like Dark Knight, Morse or Black Swan (advance versions), are quiet expensive, sometimes reaching 300$. This could be explained by the originality of their visuals and their relatively low print run.
As you can see, the value of movie collectible is dependind on many variables, which make it sometimes difficult to evaluate. But not impossible, with a method.
Therefore, to assess the value of a document, you have to collect as much reliable information as possible about similar documents. You must check the prices of sold documents, and prices currently asked for similar documents.
Make sure to compare really similar documents (year, country, size, condition..), and to clearly identify the differences. The condition of the document, for example, is essential : a perfectly preserved document will obviously have a higher price than a similar document presenting defects.
Once these information collected, focus on prices commonly recorded. Maximum and minimum prices should be ignored if they seem extreme, because they are certainly related to an unusual market behavior, for example because of a lack of buyers. Note that the country of sale is also affecting prices: some posters are very valuable in U.K, when totally unnoticed in France, and conversely. So it is crucial to take it into account.
All this research work should allow you to assess the value of your document. And if it seems too tedious, another simple solution is to consult a professional.