If Sins of a Solar Empire: Rebellion generated the same gross revenue on the Epic Games Store, Ironclad Games Corporation, Stardock Entertainment would keep approximately $7,455,792 - that's $1,620,503 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $880,335. 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 $5,835,289.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.