It is impossible to arrive at an exact value for any given stamp. The variables are infinite and constantly in flux. I am not at all surprised with the interest in the stamp. I think the grade could have easily been a 90J. Perhaps it would have been with lighter cancels.
Actually Winston, one can get an excellent feel for what is an appropriate price range for a given stamp, in particular a common stamp such as this. Try Siegel Powersearch for a start (which in this case will not provide 85 results).. You do need two bidiots however to pump up the price. As far as 90 goes, should've, would've, could've. It is an 85, not a 90. I will note that eBay is NOT a place to find appropriate price ranges. LOL!
FYI - Here are some Siegel realizations for MOG examples of this stamp in the grade of 85.
There are no rules for what a stamp HAS to sell for. Beauty is in the eye of the beholder.
P.S. The Siegel search, as well as SAN are close to useless for mid- and low-value stamps, as the chance of singles cataloguing below about $250 as singles rather than parts of balance lots is very small. The populations are too low to draw any meaningful conclusions from.
Rev - You completely miss the point. The point was to make a comparison to what has been purchased in the past at what price point. In other words, how much more stamp went for the same range of dollars. And nobody has said that a stamp HAS to sell for "x". This is a simple thread about taking note of what would seem to be very enthusiastic bidding on a given item.