If System Shock 2 (1999) generated the same gross revenue on the Epic Games Store, Nightdive Studios, Irrational Games, Looking Glass Studios would keep approximately $1,946,457 - that's $493,594 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $129,062. 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 $1,452,864.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.