I'm already aware, as per Are there price lists for magic items, such as the Bag of Holding?, that 5e was designed with the assumption that magic items are not to be bought or sold willy-nilly. This doesn't happen to be true in my setting.
I'm also aware, as per Do magic items really cost at least as much to create as they do to purchase?, that crafting new magical items is not a worthwhile effort in settings where magic items can be purchased - creating an item is more expensive than outright buying it.
My issue is different: What if the value of a non-magic item is higher than its magic equivalent? In my specific case, I had an issue with the price of Adamantine Plate Armor and the price of regular, unenhanced Plate Armor. DMG page 135 sets up price ranges for categories of items: Uncommon items would be priced in the 101-500 gp range. Adamantine Armor, which can be any sort of Medium or Heavy armor apart from Hide, is one such item. But Plate armor (which is a sort of Heavy armor) costs 1500 gp (according to PHB page 145).
My ruling on the spot was that the 101-500 gp range was for the reinforcement only, and not for the armor itself, and thus the Adamantine Plate armor would be priced at around 2000 gp. Is there another way of coming to an answer that I didn't consider?