If Mafia II (Classic) generated the same gross revenue on the Epic Games Store, 2K Czech would keep approximately $34,837,362 - that's $5,748,701 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,614,186. 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 $27,616,084.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.