I think that one possible interpretation of the recent popularity
of digital cameras is that most people are satisfied with lower
quality than I thought they would be. Consequently, the
desire to produce quality 20x24" prints might be for your own
satisfaction. Even more to the point, maintaining a rate of a
few hundred 20x24's per year might be a high estimate.

Even if it is not, I doubt that the cost of paper and ink is much
less than the cost of B&W photo paper and chemicals
(particularly if you mix your own developers).

Doubtless some adjustment in the price and performance of
high end digital backs will be seen over the next years. Still,
economies of scale need a large number of users; the
highest quality is likely to draw only a few users.

Another point to consider is that such an expensive back
would probably entice you to explore color, as well. If you
really only want B&W why not wait a while and see where the
market takes us.

Still, it is hard to argue against trying the latest new gadget.