Why is it that the Nikkor 18-200mm is so much more expensive than the Tamron 18-200mm?

Because the Nikkor lens is £517 and the Tamron is £153

Nikkor lenses can last for decades and still perform as designed. They are also more robust and are moisture and dust resistant.

If you are going to use your lens for a few decades or if you will be using it day in and day out, the added investment to buy a Nikkor lens is going to be worth it in the end.

Nikon make a way better lens. The Tamron is junk.
You will be unhappy with the Tamron. Get the Nikon!
The parts on the Tamron will ware out faster, and the focus will not be as good.
Nikon also puts better glass in there lens.
It has been proven that a Nikon D4 with a cheap Tamron lens will take bad pictures no mater what.
I watched the whole video and got to see the sample images. Nikon is way better.
You will regret getting the Tamron after a few months.
That is why the Tamron is cheaper. I almost bought a Tamorn lens.
I'm so glad I did not. I got the Nikon and I'm really happy.
the Tamron was just not sharp and had a lot of distortion and stuff.

I'm sure that Nikon will be glad to tell you that Tamron knows the value of their lens. Likewise, Tamron will tell you that Nikon is cashing in on their name.

It is a big deal that this Tamron lens does not have VC (vibration control). And it does reduce the manufacturing cost of the lens.

Actually, Nikon has two 18-200mm lenses, a 2005 version and a 2009 version. Here are the three specs side by side
http://www.dpreview.com/...-6p3_di_ii

The Nikon is sharper, distorts less, faster to focus and has less in the way of chromatic aberrations. It has less light falloff at the edges and less vignetting. The nikon also has a gret vibration reduction system which the Tamron totally lacks. That VR makes a LOT of difference in how slow a shutter speed you can use without a tripod. It is a better lens in just about every way.

If you want something in between the Tamron and the Nikon in both price and quality, take a look at the Sigma 18-200mm OS HSM zoom. The OS stands for Optical Stabilisation, so that makes it much better than the Tamron for not too much more money. The HSM version is a Hyper Sonic Motor which works properly with all Nikon DSLR cameras.

EDIT - the only real differences between the mark 1 and mark 2 of the Nikon 18-200mm lens is that the mark 2 has a lock for when the lens is at 18mm to prevent zoom creep while carrying it around and the colour of the VR symbol was changed from red to gold. There's no other discernible difference.

Superior optical quality, greater build quality and better camera compatibility.

YOU ONLY GET WHAT YOU PAY FOR when you buy camera lens
Fitting a Tamron lens on a Nikon D90 is like buying a porsche 911 and running it on paraffin
If £517 is too expensive for your budget, Try the 18-105mm VR lens. It will give you quite a bit of "Reach" and you still have the super wide 18mm