If Assassin's Creed® Odyssey generated the same gross revenue on the Epic Games Store, Ubisoft Quebec would keep approximately $144,911,308 - that's $16,282,846 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $19,624,269. 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 $115,174,904.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.