HHKB people just seem to be very vocal*. I think the endgame board for most people is probably something they've built - thus each one is a one-off.
I don't think the HHKB would be the most popular endgame. It's 60%, which a lot of people don't like. In addition it has a layout even unique to 60% keyboards - a layout a lot more people don't like. As to the cost, it's just not that out of line. No-one gripes about the cost of a Filco, which is pretty close. A HHKB, or other Topre boards, here in the US, costs about 1.5 the cost of a good off-the-shelf MX board (Leopold, Ducky, WASD, etc.). And yes, IMO, a Topre board is worth 1.5 MX boards.
But, again, I think most endgame boards are one-off which people have built so you don't hear about them so much. Some people search a long time for just the right case and other parts to make their ultimate board.
*Disclaimer: I own two.