If Warhammer 40,000: Dawn of War II - Anniversary Edition generated the same gross revenue on the Epic Games Store, Relic Entertainment, Feral Interactive (Mac/Linux) would keep approximately $14,533,701 - that's $2,749,297 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,845,505. 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 $11,465,444.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.