I don't buy it. The development, tooling, and manufacturing cost of a late model hemi intake manifold can't possibly be that much higher than those costs are for a Magnum 360.
Now, demand might be something else. The Hemi's haven't been around as long as the 5.2/5.9L Magnum, so they're aren't as many of them. But there's getting to be a lot more of them out there, just look at the production #'s (or your local wrecking yard). And they're both smog engines, so the market for aftermarket performance parts should be about the same.
I certainly don't see a reason that a carb manifold for a Hemi should cost $800. If anything, the market (demand) for Hemi swaps is being limited by the cost of the aftermarket swap parts, even more so than by the availability and cost of the engines.