If Fallout 3: Game of the Year Edition generated the same gross revenue on the Epic Games Store, Bethesda Game Studios would keep approximately $21,407,673 - that's $3,764,770 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,782,864. 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 $16,933,376.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.