If Alien: Isolation generated the same gross revenue on the Epic Games Store, Creative Assembly, Feral Interactive (Mac), Feral Interactive (Linux) would keep approximately $50,426,020 - that's $7,693,275 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $6,739,912. 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 $40,016,153.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.