If Life is Feudal: Your Own generated the same gross revenue on the Epic Games Store, Bitbox Ltd. would keep approximately $6,009,312 - that's $1,324,632 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $683,088. 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 $4,684,680.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.