If The Killing Antidote generated the same gross revenue on the Epic Games Store, MetalStar Studio would keep approximately $3,638,492 - that's $839,692 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $359,794. 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 $2,798,801.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.