If Age of Mythology: Retold generated the same gross revenue on the Epic Games Store, World's Edge, Forgotten Empires, Tantalus Media, CaptureAge, Virtuos Games would keep approximately $11,601,898 - that's $2,316,189 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,445,713. 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 $9,133,328.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.