The D800 is huge news. Expect it to be out of stock for months after its launch. Of course, people will want to get the grip as well, so Nikon sees this as a chance to pocket a little more cash. I would expect the price for the grip to reach reasonable levels once the first wave of buying frenzy cools down.
I don't like this pricing strategy either, and personally, will wait for the prices to cool down before I will consider buying one.

The answer easy, The D800 is more expensive and is the best high res sensor camera Nikon makes so the grip is more expensive. You always get charged a premium for accessories that go with top of the line products. Especially when those products are new, extremely desired, and the manufacture is likely to be unable to meet the initial demand. Give it 6 months minimum or more likely a year and the price will probably come down a bit. It all depends on how many people let their excitement for the camera overcome their judgment enough to actually be willing to buy a grip that has such an unusually high price.