The development and certification costs alone would require that the
selling price be in the VLJ range.

Not so sure about that, though. Diamond's DA42 Twinstar lists for under
500k.

I have not looked closesly, but I believe there is a large parts
commonality between the DA-40 and the DA-42.
This is similar to the Adam 500 and the Adam 700.
Anytime you can leverage existing parts into a new design, you reduce
costs.
The seat attach fittings on the current King Air's are the same as the
Twin Beech model 18's.