There is none.
Some tier one items may be a lot more valuable than even the most expensive tier 5 item. There is no reason for this other than to say the value is what the market will bear.
The price varies in different parts of the world due to the cost of access and availability. Also, it is a seller market with the idea of “buyer beware” as to how much they are willing to pay. Although many items have suggested values, there is no set price per item or tier.
The guide manager (GM) controls the economy for their table… There is no requirement for an NPC to buy the items the players may wish to sale. The NPC can always tell the players that they do not have enough money to buy an item, or they do not deal with items of that type or level.