I generally would agree that the D&D world should try to simulate its own economy rather than a medieval one. I think that if the planes are going to interact with the mortal world at all, and have creatures that can be interacted with and killed, they should be probably be traded with. Sigil would be a good way to do it, or the City of Brass (which I like better than Sigil myself). Now a good question would be what the material world could offer these far more wealthy and cosmopolitan places. Certainly not raw materials (more abundant in the elemental chaos), heck perhaps they wouldn't even desire gold. Really, the only thing that I can think extra-planar creatures would want is human (and other mortal) slaves. Otherwise possibly they might want wood and other organic materials. Still, not a very profitible relationship.
However, it is largely beside the point. The price of D&D magical items is not based on what gives you personal power in the mundane world (which is full of dirt farmers) but what helps you hit the defenses of dragons and extra-planar monsters. It is for weaker creatures to kill stronger creatures, which are impervious to creatures weaker than you. Millions of common soliders are useless against Orcus. Which is generally why, in my opinion, magical blades should be found in dungeons with a prophecy to kill Orcus, rather than bought for huge sums of money.