As of Monday, June 5, it takes around 280,000 gold to buy.

a WoW Token. Each of these Tokens corresponds to $15, so it would take around 1.4 million gold to buy enough Tokens to afford Diablo 4, with taxes included.
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Related: Diablo 4 https://www.lolga.com/diablo-4-gold Nightmare Dungeons, explained