GnomeWorks
Adventurer
I'm not positive, but are you limiting your model to a standard medieval society? I haven't participated in an epic-level D&D game thus-far, but of those I've read about, you tend to expand beyond the base society at those levels.
In the economic model I'd like, the sort of crazy epic stuff that happens in high-level D&D doesn't really happen... the closest thing to what I'm envisioning, in terms of D&D, I guess, would be E6.
A noble or even a king may not be able to spend 3 million gold on a magic sword...but perhaps a Hound Archon could purchase one from the City of Brass, etc. etc.
Just to let you know, my mind said "ARGH!" as soon as I read that.

No, I haven't considered extraplanar things so far as my thoughts on in-game economics, and I have little to no interest in going there. While your example might bring some amount of validity to 4e economics (presuming that groups always follow the heroic-paragon-epic thing of going to increasingly distant planes, which may or may not be the case), it's not something I'm about to consider.