If Back 4 Blood generated the same gross revenue on the Epic Games Store, Turtle Rock Studios would keep approximately $85,929,957 - that's $10,920,905 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $11,581,358. 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 $68,257,920.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.