If Total War: ATTILA generated the same gross revenue on the Epic Games Store, CREATIVE ASSEMBLY would keep approximately $25,872,388 - that's $4,424,330 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $3,391,689. 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 $20,484,854.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.