The difference is in camera build quality, the D200 being the better of
the two. The D30 has better high ISO performance, but the cameras are
equal under normal ISOs. The big difference is that Nikon can't keep
the D200 on the vendors shelves as it was their 1st mid grade camera in
4 years, and they did a good job, with a few teething issues. The D30
is a minor upgrade of the D20, so D20 owners have very little reason to
upgrade. So Canon keeps the price point low to sell cameras, Nikon can
afford to keep the price high on the D200 since it is tough to find
one. In fact there is a rumor that Nikon is going to raise the price of
the D200.

Tom

The 30D is in the same segment as the D70s (which costs about a third less),
the D200 is roughly equivalent to the 5D (which costs twice as much).

Because it's a far more cheap construction.
For electronic devices like DSLRs the actual construction cost makes up a
relatively small part of the total price, the component parts a far greater
part.
The component parts of the 30D are lower spec (and maybe lower quality) than
those in the D200.

Both cameras are worth what you are willing to pay for them.
This may or may not bear any relationship to the cost of production of the
item.
Just how much do you think that Ralph Lauren Shirt that somebody paid $50
for actually cost to produce and market? What do you think the seamstress in
a third world sweatshop was paid? Why does the same Ralph Lauren item cost
less at Costco than at a Polo outlet store?
Are the components in a Lexus any more costly to assemble than the
components in a Buick?
Do you really believe that they are?
Look at Canon's marketing techniques, of which the 30D is a prime example:
as noted this is not really a "new"camera. Are the Pixma printers that use
the same inks as i9x printers several generations ago really "new" printers?
At least the D200 is a substantially new camera design, but its first
generation successor is likely to be merely a modification of dubious value,
ala the D70s vs the D70. And I assure you that Nikon knew what those next
generation modifications would be prior to releasing the D200.
This is how marketing works in our bizarre economic system where the
"market" decides all despite ever diminishing natural resources and the
increasingly insurmountable chasm between the haves and have nots.

The price of photographic equipment (or almost everything) these days is not
determined by one piece of equipment being technically "better" than the other
and in certain cases is not even determined by the cost of manufacturing. The
principle "you get what you pay for" ceased to work a long time ago. The price
today is defined by the marketing strategy chosen by the manufacturer. In this
particular case Nikon decided to price-postion D200 with regard to 20D (and now
30D) so that it creates an illusion of a "better" camera, at the same time
playing the "hard to get" game: the D200-s are indeed in relatively short supply
now even though the demand is not that hight compared to that of 20/30D. This is
a rather old well-known standard marketing strategy. Nikon chose to follow it
apparently hoping that it will work out to a better profit. Only time will tell
whether they were right.

I don't think they were hoping to compete with 5D at a significantly lower price
level, since the technical qualities of D200 are only adequate to compete with
20/30D.

--
Best regards,
Andrey