Processing 13+ channels does not come cheap. Even Monoprice's new pre/pro (no power amps included) is coming in ~$4000 and it is one of the least expensive options. 13 power amps does not come cheap. Engineering (circuitry layout/cooling/power & noise handling/etc.) this beast is not cheap.
8500 adds 20lbs in weight over the 6500. Denon did not just take a 6500 and plop 2 additional amps into the device. There is obviously more going on than just the 2 channels.
In addition, Denon's flagship receivers are designed with a multi-year shelf life whereas the lower models are designed to be refreshed every year (this year being different due to HDMI availability). A lot of forward-thinking goes into something like that and it starts much sooner than a product that gets refreshed on a yearly schedule. I am speculating here, but I would not be surprised if there is a different, higher-end design team for the flagship products vs the 6500 and lower.
Check out photos of the rear panels and internals for each model. They are both very different.
I am not sure what links you are hoping to find or have someone provide. Denon does not provide a BOM (Bill of Materials listing every part and the cost of each part) you can download. Even if so, that would not cover engineering/production/packaging/marketing/shipping/etc. costs (yes, every one of those steps and more is calculated in the final price).
Then, there is the almighty markup. For every product, the higher level has a bit more markup. Think trims in the car market. The base model may be $15000. When you step up to the top of the line, you may be looking at $30,000. Is there really twice the car value there? No. But, to some consumers, they want/need the options and conveniences provided.