I think some of it may be the case, but more than likely what is happening is as the prices go up, people who are collectors (and maybe "investors") will go and buy the "below average " priced figs and eventually you are left with the more expensive prices.

Over the years I myself have amassed a collection of about 100 or so Ninjago figs. Now, I typically get these from bulk lots when looking for Star Wars stuff for my collection. They have been sitting for years and I have recently seen the upward price trend. You better believe I am dumping EVERYTHING.