You know what else would require you to know how much gold is in your game and hundreds to thousands of other details?
The price of hiring workers (2 gp per day for skilled workers, PHB pg 159)
The price of a merchant galley (30,000 gp, PHB 157)
The price for fine clothing for a fancy dinner party (15 gp. PHB 151)
The Price of a Greatsword inlayed with silver (150 gp, PHB 149-148)
The Price of Full Plate Armor (1,500 gp, PHB 145)
Seems WoTC had no trouble pricing mundane items, mounts, vehicles, airships, workers, weapons armor... and yet it was far to difficult for them to even begin conceiving of pricing magical items? That seems... like a load of BS.