Sphere of exchange

I felt that the sphere of exchange was relating to the different layers or types of merchandise being exchanged.  They said there were 3 tiers of merchandise in which they categorized, whether it be from humans through bridal exchange through something as minuscule as chilies.  The chain of motions that the merchandise travels in is also a very likely thought in which the term "sphere of exchange"is meant.  What i found interesting about the reading was how they value different items.  How they determine what is worth the highest exchange.  Brass for one, they said the price fluctuated a lot, it started at the top and is slowly decreasing in price.  I wonder why bronze is such a powerful exchange in the culture.  I mean i know mainly because of the jewelry factor, but being from a materialistic society its hard to wrap your mind around.  All in all i found this article pretty educating about the transfer or exchange or properties within the TIV population.