If ULTRAKILL generated the same gross revenue on the Epic Games Store, Arsi "Hakita" Patala would keep approximately $70,472,419 - that's $9,515,674 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $9,473,512. 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 $55,962,152.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.