If Mafia III: Definitive Edition generated the same gross revenue on the Epic Games Store, Hangar 13, Aspyr (Mac) would keep approximately $41,279,071 - that's $6,700,317 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,492,601. 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 $32,740,170.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.