If STAR WARS Battlefront II (Classic, 2005) generated the same gross revenue on the Epic Games Store, Pandemic Studios would keep approximately $11,923,795 - that's $2,363,742 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,489,608. 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 $9,389,383.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.