If Mount Your Friends generated the same gross revenue on the Epic Games Store, Stegersaurus Software Inc. would keep approximately $1,042,052 - that's $308,602 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,734. 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 $733,451.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.