Paul:

Have you explored the possibility of a teleconverter lens. Nikon offers them in 1.4X, 1.7X, and 2X. I don't have any first hand experience with Nikon, but their teleconverters receive pretty good reviews from users at both B&H and amazon. You'll lose 1 to 2 f/stops and they are not compatible with all existing lenses, but at around $400 - $500, it would be much cheaper and easier to use than a $2,000.- scope.

I have used a Zuiko EC-20 (2X teleconverter) with my Zuiko f/2.8 - 3.5, 50-200mm zoom on an Olympus E-30. It does pretty well in good light with very negligible loss of sharpness & detail at the long end. Of course, given Oly's 2X crop factor, the resulting 800mm Effective Focal Length (in 35mm terms) makes a tripod absolutely mandatory to avoid camera shake--even with in-body image stabilization. It also tends to focus hunt quite a bit in less than optimal light.

As a result, I recently purchased the EC-14 (1.4X) and, although I haven't tried it in the field yet, so far I am pretty happy with it based on my backyard tests. Tack sharp with no loss of detail and I can hand hold it at the long end without having problems with camera shake. No problems with focusing even in poor light.

Legacy glass is another low cost alternative. There are a lot of excellent quality prime telephoto lenses out there for quite reasonable prices. Of course you'll sacrifice some or all of your auto-focus & auto-exposure capability, which may or may not be a significant problem for you. I've played around with some legacy glass and it has been fun, but . . .

I'd be interested to hear what the Nikon shooters here recommend.

Jim