Reading through the thread, although I don't own an HHKB, I felt the need to summarize why it might be valuable, and to attempt to justify why it might cost what it does.
Layout: no windows key (feature or bug depending on preferences), placement of ctrl, backspace etc.
Switches: capacitative is cool. Patent leads to higher prices.
Rubber: superior grade vs. ordinary rubber cup.
Key Weights: 45g uniform. Deemed ideal by some people.
Dip switches: offers some limited hardware-based flexibility.
Small form factor: takes up less room on desk, easier to carry, allows for optimal positioning of mouse or trackball. Lighter than other topre boards.
Aethetics: red esc key, possibility of blank keys, minimalism.
Quality build: as shown by tear-down by ripster in other posts. Importance of quality control to Topre corp.
Japaneseness/Geek cred/Oneness with cup-rubber: other intangibles
Overall typing experience: important when you really will max out the rated number of keystrokes of a board
Comparative cost of other baskets of goods: trips to RSI docs (or should I say first five minutes with an RSI doc), cost a nice dinner out (in many cities), cost of a parking ticket (in some cities)
If anyone has anything to add, I hope you will let me know.