If you have a D5000, you will have to manually focus the 85mm f1.8D lens. To get an Auto Focus lens for this level of camera would require the 85mm f/1.8G lens. It is important to know whether your camera has an autofocus engine or not.
It's worth every penny, especially if you take portrait shots. I do not have the 18-200, but I have the 16-85 VRII. While the 16-85 is a great lens for outdoor shots, landscape and group photos, the quality of portrait I get from the 85 1.8 (or any prime lens for that matter) is way above any all around zoom. Most pictures I take with this lens are worth keeping.
However, you're the only one who can decide whether it's worth it to you. How much do you value control over out-of-focus background (bokeh) or having a "3-dimensional" look? Do you like being able to shoot indoors and in other low-light situations without using a flash or high ISO settings?
If these characteristics or important to you, then the $450 may be worth it.
I have both the 18-200 and the 85 mm f/1.8 among others. These two are the lenses I most use (I photograph people and places). The 85 mm is great for head shots, particularly indoors without flash. If $450 is a reach, try eBay where you'll see lots of them.