If South Park: The Stick of Truth generated the same gross revenue on the Epic Games Store, Obsidian Entertainment would keep approximately $41,579,769 - that's $6,744,739 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,533,605. 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 $32,979,362.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.