If Sniper Elite 4 generated the same gross revenue on the Epic Games Store, Rebellion would keep approximately $68,353,967 - that's $9,323,088 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $9,184,632. 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 $54,277,020.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.