If A Total War Saga: THRONES OF BRITANNIA generated the same gross revenue on the Epic Games Store, CREATIVE ASSEMBLY would keep approximately $9,647,122 - that's $2,027,416 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,179,153. 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 $7,578,392.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.