How much would it cost to craft a magic item that bestows the effects of a level 3 spell whenever it is worn? The item can be something simple, like a ring or an amulet; its nonmagical properties aren't important.
The guidelines (they're not hard rules by any stretch of the imagination) for creating new magic items appear on page 284 of the Dungeon Master's Guide.
The table "Magic Item Power by Rarity" advises that items creating a 3rd-level spell effect once per day are at least Uncommon. However if the effect is continuous, the advice is that the item rarity should be much greater -- no less than Rare. Assuming the 3rd-level effect to be a least a step above this baseline, it would be at least a Very Rare item.
The suggested price range for Very Rare items (DMG page 135) is 5,001-50,000 gp. The cost for crafting them (DMG page 129) is 50,000 gp and 2,000 days (5½ years) of work. It's suggested that characters should be roughly level 11 before the GM introduces items of this potency.