Evaluating the worth of posters is a tricky business and if you ever understand it you should go into the business full time. Market forces play a vital role - namely supply and demand.

Supply - Dictated by number of posters initially printed, number of posters surviving (often directly related to age), and number of posters being sold on the market.

Demand - Dictated by social importance/value of the film, word of mouth, rumors, and more rumors. Dealers are often surprised at what the public at large and the collecting community find worthwhile to buy. Forgetable films usually but not always have forgetable posters. Cult films, Oscar winners, independant films are often good bets.

However, more important than either supply or demand is a third factor:

Condition - A rare, well sought after poster that is in mint condition will acheive excellent prices. However, the same poster that is then ripped, corners missing, dented, faded is often worthless not matter how important and rare the title. When evaluating prices of any poster, be certain to evaluate condition.