If Serious Sam 3: BFE generated the same gross revenue on the Epic Games Store, Croteam would keep approximately $12,060,952 - that's $2,384,004 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,508,312. 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 $9,498,485.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.