If Warhammer 40,000: Mechanicus generated the same gross revenue on the Epic Games Store, Bulwark Studios would keep approximately $8,491,711 - that's $1,832,395 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,021,597. 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 $6,659,316.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.