If Warhammer: End Times - Vermintide generated the same gross revenue on the Epic Games Store, Fatshark would keep approximately $13,578,018 - that's $2,608,116 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,715,184. 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 $10,705,242.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.