Valuation is a very fickle subject. In my opinion, there are three main sources of valuation, the sellers (as in a poster dealer), the owners (as in what you think it is worth to you), and free market (auctions).
Now there are multiple factors to consider when assessing value.
Rarity - First of all is rarity. How rare it the poster? Does it come up for sale 20 or 30 times a year? Has it not been available for the past 3 years? How many examples are there suspected to exist?
Condition - The second factor in value is how well the poster has been treated. Is it beat up? Tears missing pieces... Is it mint? (not likely for older posters)
Reproductions - Sadly if a poster has known reproductions this can greatly effect value. It is, at times, very difficult to determine authenticity and this will drive down value because there is risk involved.
How do I assess value? - I tend to look at multiple auction histories (like Bruce's emovieposter histories) and retail websites. Remember for a retailer to clear a profit, at most times they are working on a 50% or greater margin. So you have an original "blah" and "xyz" is selling it for $500... Most likely he only paid $200 or less in an auction or lot purchase...
At the end of the day, a poster is only worth what someone will pay for it...
Some interesting discussions on valuation: