I was looking at some of the items' prices in the DMG- specifically the +1 and +2 shield. Per the FG DMG. A +1 shield (Uncommon) is listed at 112gp and a +2 shield (Rare) is listed as 838gp. If my memory is correct, in the PHB or DMG it says that rare items have a minimum of 5000gp in value. Another issue is the Adamantine Plate Armor. It is listed as 807gp, and it is considered uncommon. The regular plate is listed as 1500gp in the PHB.
Where do these prices come from and should they be considered official since they are listed in the licensed content? Or, should these prices be fixed to align with the posted price ranged?