If Assassin's Creed 2 generated the same gross revenue on the Epic Games Store, Ubisoft Montreal would keep approximately $23,113,026 - that's $4,016,697 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $3,015,413. 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 $18,289,907.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.