It's supply and demand. There are plenty of silent posters where there are only one example known, but NO ONE is particularly looking for it, so it keeps selling for a relatively low price.
If there are 500 of an item, but 501 people want to own it, the price will rise. When those numbers shift, the price will fall. But in the case of Star Wars, that may be many years off (and many dollars higher than current prices).
Bruce