If Software Inc. generated the same gross revenue on the Epic Games Store, Coredumping would keep approximately $3,936,162 - that's $900,579 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $400,386. 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 $3,035,584.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.