If FINAL FANTASY XV WINDOWS EDITION generated the same gross revenue on the Epic Games Store, Square Enix would keep approximately $38,967,563 - that's $6,358,845 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,177,395. 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 $30,901,471.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.