If Batman: The Enemy Within - The Telltale Series generated the same gross revenue on the Epic Games Store, Telltale would keep approximately $3,301,220 - that's $770,704 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $313,803. 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 $2,530,516.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.