If Stronghold Crusader HD (2012) generated the same gross revenue on the Epic Games Store, Firefly Studios would keep approximately $3,537,001 - that's $818,932 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $345,955. 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 $2,718,069.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.