If Steep™ generated the same gross revenue on the Epic Games Store, Ubisoft Annecy would keep approximately $13,809,518 - that's $2,642,315 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,746,753. 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 $10,889,390.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.