Commodity pricing has been broken for a long time, in that the shopkeepers would never offer to buy back an item for more than its base price. This created the problem that asking prices would inflate while offer prices would not, and therefore it would never be worth anyone's time to sell anything back. One place this is particularly problematic is with glass bottles, and the slow rate of sand mining.
With these changes, we are making it so the shopkeepers will offer significantly more gold when buying back commodities with high prices. This will make it worthwhile in many cases (like glass bottles) to sell these items back to shopkeepers. It also opens up opportunities to find creative ways to profit by trading items around, either by simple buy low/sell high, or by resource gathering and crafting coupled with trading.
I would not expect its price to climb as high as some think it might. It's not too difficult to mine, and if the price gets too high then it will be worthwhile to mine some just to sell to the NPC shopkeepers.
*stocks up on arrows and bolts to make a ton off Nix Again*