If The Bureau: XCOM Declassified generated the same gross revenue on the Epic Games Store, 2K Marin would keep approximately $5,904,286 - that's $1,303,149 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $668,766. 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 $4,601,137.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.