If Mafia: Definitive Edition generated the same gross revenue on the Epic Games Store, Hangar 13 would keep approximately $76,030,975 - that's $10,020,998 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $10,231,497. 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 $60,383,730.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.