If Black Desert generated the same gross revenue on the Epic Games Store, Pearl Abyss would keep approximately $21,436,641 - that's $3,769,049 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,786,815. Epic charges 0% on the first $1 million in annual per-app revenue and then 12% on the remaining amount. On GOG (30% cut), the developer would earn approximately $16,956,419.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.