I know those shops by the British Museum you mention, and like many shops here in Paris, they post prices that simply have no relation to true market value. Of course, the value of something is whatever someone will pay for it, and maybe if they keep it on the shelf long enough someone will buy it, but that means carrying inventory.
The finder you've got is nice, if you've got the right camera to use it with. I can't see the bottom, but I imagine all you see is a little ball -- no manual parallax mechanism. If so, that's because these model finders were intended to mate with mechanisms built into the cold shoes of various late-model Canon rangefinder cameras -- like my Canon L-1. On a proper camera, it results in automatic parallax adjustment as you focus. On a not-proper camera, you can't adjust the finder. So it's valuable to some users, and not so valuable to others. Personally, I think it should sell for about $110 or so, to a motivated buyer.