M15s often sell for over a grand.
Wasn't there a NIB one that went for $1800 or so last summer?
The gray (Olive?) case industrial IBM's can sell for a lot. NIB M13's (with black/white keycaps) also can go for a couple hundred. Industrial Model M SSK's are also pretty rare.
The cherry MX5000's also sometimes go for a huge amount.
I've never seen one for sale, but I imagine a really rare keyborad like a knight keyboard would sell for even more.
Other than "unique features" such as that, lower serial numbers, older dates, etc. make IBM keyboards more expensive. The one in question is in good shape, is old (february of 87!) and is industrial, all factors that add to the price. Ofcourse, sometimes they aren't posted or listed. I have a 122-key model F that appears to have been manufactured during the first month of production, though this wasn't listed on the auction, so I was able to get it pretty cheap. I don't know what it's worth, though.
As far as the "holy grail" of keyboards:it depends on your prefrence. Some like hall-effect vintage space cadet keyboard or the knight keyboard, which are ingrained in computer culture. Some are after the korean custom keyboards (356 series, etc.) Some like the rare aluminium case filco, or that HHKB with hand-laquered wood keycaps.
Some like the exotic IBM's:
http://www.clickykeyboard.com/2010/jul17/010.jpg