I take your point, a bit, but the cost of re-engineering everything would have moved this well out if the $8,500 realm. The Canon 1Ds back in 2002 cost $8,000, a new 1DX is over $6,000.
Anyway here are few numbers for the number crunchers.
Pentax 645Z - 6.1 x 4.6 x 4.8" / 15.5 x 11.7 x 12.2 cm $8,500
Hasselblad H5D - 6.02 x 5.16 x 8.07" (15.3 x 13.1 x 20.5 cm) $13-44,000
Leica S - 6.3 x 3.1 x 4.7" / (16.0 x 8.0 x 12.0 cm) $22,000
Mamiya RZ22/33 - 6.5 x 6.1 x 5.2" (16.5 x 15.5 x 13.2 cm) $11-18,000
The issue is I'd guess not the cost of the camera as I can't see why using a smaller flange distance that better matched the smaller mirror needed for a 44x33mm sensor would increase costs greatly but rather lens cost/support.
You can see that not only aren't there many new lens options(25mm, 55mm, 90mm macro) the cost of them is more inline with other modern high end lenses, Its only the only cheaper film 645 lenses that would allow the Pentax 645 system to be a budget option.
Still I think a better route could have been to go with a smaller flange distance and just offer a converter for the larger lenses. For the rich amature market especially I think the Pentax's massive block form factor doesn't really sell itself well.