I'm probably in the minority on this, but a unicorn poster to me actually has two definitions. First, there are just "my unicorns", then there are the normal unicorns.
My personal unicorns are based purely on cost, nothing else. If there is a $10,000 poster out there that I really, really, like and want, it's a unicorn for me. This poster is technically, acquirable. Maybe, just maybe, if I saved diligently for 10 years strait, I could very well acquire it. So is it a stretch? Absolutely. But, indeed possible. Therefore, "my unicorns" are at the very edge of my financial reach.
Regular unicorns however have more layers of criteria. My thought process towards normal unicorns is that these poster for starters need to be much more then just rare. For they should always be financially elusive to average person/collector. Therefore, besides being rare, the poster should also be somewhere around 16K+, and of course be desirable as well.