If XCOM: Enemy Unknown generated the same gross revenue on the Epic Games Store, Firaxis Games, Feral Interactive (Linux) would keep approximately $18,201,951 - that's $3,291,197 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,345,721. 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 $14,383,370.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.