From my understanding, the resources in minecraft ARE limited. There's only so much of it. Glowstone is extremely limited. Some resources are readily available. But still, it takes time and effort to mine something as simple as dirt. Even dirt is limited, or water, or cobble, or wood. We don't get things out of thin air. We have to find and mine them, right? If everything was unlimited, there wouldn't be a price tag. Everything would be free. There would be no buyers and no sellers.
This doesn't actually work.
Think of it this way: If the map in Minecraft was infinitely large (Which it essentially is in SP), and we started using an infinitely large map on the server, that means infinite number of chunks = infinite number of diamonds. Does that mean diamonds = free? No. Because You pay for the resource AND the time and effort it took to find it, mine it, and bring it to you.
From what I learned in my high school economics class, the price is determined by the intersection of the supply and demand curves. If demand increases, the entire demand curve shifts to the right. The buyers will pay more for the same quantity. If supply increase, the entire supply curve shifts to the right. The seller can provide more quantity at the same price. This usually means that costs have gone down for the supplier. What's going to happen in this nether wipe is there will be a sort-of surplus in glowstone - a supply increase. Since there's going to be so much of it, wouldn't price decrease? In response to a supply shift, quantity demand will increase. The demand curve won't shift. The equilibrium price and quantity will just change.
Except that more supply does not necessarily equal cheaper price. If all the glowstone was taken by a single group of people, they could raise the price to fairly astronomical heights and still have buyers, because they control the market and people will need glowstone. It's like owning EVERY property in monopoly.