I think a PPC-based one would be much more expensive though and not directly competitive
This is a good quote, and applies to anything, including a workstation.
Why the heck would someone pay crazy money for a board that runs an OS (Linux, Haiku) that'll run better (performance and support-wise) on hardware a fraction of the cost?
At least with AmigaOS and its kind the hardware choices are limited so people are more likely to consider an overpriced board, but as has been mentioned before, the market is small.
That said, I'm sure there are probably at least 500 people in the world with money they'd blow on such an item if for nothing more than to kill some time. It might just take a couple years to find them all.