If Imperator: Rome generated the same gross revenue on the Epic Games Store, Paradox Development Studio would keep approximately $18,890,985 - that's $3,392,986 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,439,680. 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 $14,931,465.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.