If Assassin's Creed® Origins generated the same gross revenue on the Epic Games Store, Ubisoft Montreal would keep approximately $95,554,973 - that's $11,795,907 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $12,893,860. 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 $75,914,183.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.