Since the item is acquired from the Dark Auction, the price may fluctuate due to supply and demand. If no one at the auction has an interest in the item, the player may be able to bid significantly less than someone in a different lobby where multiple players desire the item. This appears to be a popular end game accessory because of the damage reduction against withers, which includes Necron.
Trivia[]
There was a bug where you could stack the Wither Artifact with the Wither Relic to get 45% reduced damage taken from Withers.