As Bruce and Jay have explained, the value of a poster is essentially the average of available data minus the outliers, but even this method flawed and sometimes useless. Let's take my precious Australian Empire poster for example, shall we?
If you looked up the 1-Sheet, you'd see prices ranging from below one hundred to above one hundred, but never surpassing two hundred dollars. The most recent auction on eMovie sold the poster for $180. But, one must also take into account the rarity of the poster, because much of the sales data is half a decade old (if not closer to ten years). Considering how many more people use the internet than in 2003, and the fact that people like Thierry have been destroying them, the number of potential buyers and the number of available posters has increased and decreased respectively.
While there is no sales data to indicate the poster is "worth" that much, I can't get anybody to let go of their AUS Empire Strikes Back poster for less than 3x the value as indicated by the eMovie and Heritage archives. In this case the number doesn't matter, because the new factor isn't the sales history, it's the availability as dictated by those who hold them.
This poster could be worth a thousand dollars now. The values for all of our posters are completely artificial.
Even the daybill format of the poster. In Bruce's most recent daybill auction (last week?) a scribbled on an staple-holed version of the poster came up and it sold for $200, But the sales data for this poster never puts the value of the poster over a hundred dollars. So why all of a sudden has the value has increased 4x?
Because all memorabilia is worthless, and only commands a price a buyer is willing to pay. That's why there is a CineMasterpieces, because if people pay sticker, then that's the value of the poster.