I see a paradox here. None of you seem to question why a Universal horror poster should be "worth" hundreds of thousands of dollars, yet only a very few have ever actually sold for that amount, and it is to the same tiny group of people. Yet when a $300 poster rises to $500, the buyer must be a fool, and if a year later it goes to $750, that is the height of insanity!
Back in the late 1980s I attended one of the early animation auctions, and there were old-timers in the audience who could not believe how "stupid" the buyers were. Cels from TV shows like The Flintstones with good images of the top stars were selling for $50 and the old-timers were muttering "Those are $5 cels". Meanwhile cels from completely forgotten 1930s cartoons that showed forgotten minor characters were selling for $20, and the old-timers were muttering "Those are super-rare and should be $50 cels".
Times change. Tastes change. People forget who Franchot Tone or Rosalind Russell were (along with hundreds of others) and they will pay more for Clint Eastwood posters from his best films. Just because a poster WAS $25 for a number of years doesn't mean it can't now rise to $50 or $100 or much more, and just because another poster used to be $1,250 for a number of years doesn't mean it can't fall back to $750 or $500 or even lower.