If Microsoft Flight Simulator X: Steam Edition generated the same gross revenue on the Epic Games Store, Microsoft Game Studios would keep approximately $10,453,453 - that's $2,146,533 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,289,107. 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 $8,219,792.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.