If Mafia II: Definitive Edition generated the same gross revenue on the Epic Games Store, Hangar 13 would keep approximately $14,429,013 - that's $2,733,831 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,831,229. 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 $11,382,169.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.