If Assassin’s Creed Brotherhood generated the same gross revenue on the Epic Games Store, Ubisoft Montreal would keep approximately $9,960,648 - that's $2,073,732 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,221,907. 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 $7,827,788.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.