If Farming Simulator 15 generated the same gross revenue on the Epic Games Store, Giants Software would keep approximately $3,494,594 - that's $810,258 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $340,172. 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 $2,684,336.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.