Regarding Mac Pro case design:
Actually I have to deal with either the RAF or sea containers.

I wouldn't count on OEM "Plastic" Petes surviving.
And it's not that the case is pretty. My Mac Pro has been half way around the world with me and no problems. Your "Plastic Pete" Dell or HP might not do as well.
I wouldn't say the case is a bad case. It's certainly far better than the cases
of most consumer grade machines. How it compares to the cases of other
workstations is something I'd like to read more about.
As you may know, other workstations are made from a variety of materials.
Sometimes plastic is used for some or part of the case, but also metal cases
are common.
The Mac Pros have cases largely made out of anodized sheet aluminum
(likely an alloy). The design is clever in a way. If you look at the left side,
you can see how the handles and feet have been formed by bending the
sheet.
Quite how strong this design is could be questioned. The handles and feet
do seem to be weak points, as you can see in these threads:
https://forums.macrumors.com/threads/993759/
https://forums.macrumors.com/threads/595032/
The main area of weakness appears to be where the handles and feet meet
the removable right side panel of the Mac Pro. This kind of sheet aluminum
is not as strong as, for example, certain types of steel of a similar thickness,
and so will deform relatively easily if the machine is dropped when handled.
For this reason it is important to package the Mac Pros carefully when they
are moved any distance.
Regarding Mac Pro case cost:
How costly is the Mac Pro case to manufacture? That is hard to say.
The material costs likely aren't very high, but you'd need to know the
particular aluminum alloy used to make a good guess. And then there
is the cost of processing the raw material, which could quite possibly
exceed the cost of materials.
But we can say this: the Power Macs, which had very similar cases, sold
for much less even as recently as the second half of the last decade. For
example, the entry level Power Macs used to sell for $1499 direct from
Apple:
http://web.archive.org/web/20050203023200/http://www.apple.com/powermac/
So, then at least, it was possible to sell a desktop Mac with a case like
the one the Mac Pro has for much less money.
Regarding comparisons with iMacs:
At that time, in 2005, the entry level iMacs sold for $1299. So a multiplier
of 1.15: the entry level Power Macs were 15% more expensive than that.
Today, 6 years later, the entry level Mac Pro is $2499, a 67% increase in
price over its predecessor. The entry level iMac is now $1199, so it has
a price 8% lower than that iMac from 2005. And the iMac/Mac Pro
multiplier is 2.08.
That is, where the entry level Power Mac used to be 15% more expensive
than the entry level iMac, the equivalent figure now is 108%.