If X-Plane 11 generated the same gross revenue on the Epic Games Store, Laminar Research would keep approximately $13,119,944 - that's $2,540,446 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,652,720. 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 $10,340,865.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.