If Surviving Mars generated the same gross revenue on the Epic Games Store, Haemimont Games would keep approximately $13,355,856 - that's $2,575,297 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,684,889. 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 $10,528,522.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.