If TerraTech generated the same gross revenue on the Epic Games Store, Payload Studios would keep approximately $12,567,827 - that's $2,458,884 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,577,431. 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 $9,901,681.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.