THIS is what we really need: A better way to judge relative item utility. Once we have that, we can use it for a number of purposes, of which pricing is only one.
For example, it's easier to balance things between party members when using numbers. When I (the DM) place a magic treasure, who should it be best suited for? Well let's see, CapnZapp's total item utility of 18, but Zardnaar's only 10, so the next item should probably be something Zardnaar needs more than CapnZapp.
Or, a utility number could indicate balance versus monsters. Say you've got a level 4 party, but their total item utility is 22, and the guideline says the total item utility at Tier 1 should be 0-8... that may be a clue that this party is going to run roughshod over a typical level-4 adventure.
It baffles me that spells have spell levels that people consider sacrosanct, but the idea of rating items in a similar fashion is somehow seen as impossible. On these forums, you can find plenty of examples of people debating what level a new spell should be, or whether a new subclass ability should be higher or lower level by comparing it to spells. And item rarity really does not correspond well to utility; see the discussion of "Major vs. Minor" items in Xanathar's for a good illustration of how rarity really falls down on the job. I do think rarity is useful in indicating how rare an item is, but not its overall utility or market value; for example, universal solvent is Legendary, but pretty useless most of the time.
