If Sid Meier's Civilization: Beyond Earth generated the same gross revenue on the Epic Games Store, Firaxis Games, Aspyr (Mac), Aspyr (Linux) would keep approximately $22,009,479 - that's $3,853,673 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,864,929. 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 $17,412,086.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.