One reason that it's expensive is that you're asking for pretty much all of what Nikon can pack in these days: high frame rate (and therefore high data processing rates), the largest sensor, and most of the other pro characteristics. If you shoot a D3 and a D300 together (and I have), it is very easy to see the differences - in an hour you will be sensitized to the difference in shutter delay, for example. If you shoot the D300 by itself, you won't notice. But back-to-back with the D3, D2h, D4 you will be able to perceive the difference - the timings are clearly different.
The D3 is also rated for 300k cycles, the D300 much less (half?) that.
_____ Brian... a bicoastal Nikonian and Team Member
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