If The Elder Scrolls IV: Oblivion® Game of the Year Edition (2009) generated the same gross revenue on the Epic Games Store, Bethesda Game Studios would keep approximately $9,749,643 - that's $2,042,561 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,193,133. 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 $7,659,943.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.