If The Binding of Isaac: Rebirth generated the same gross revenue on the Epic Games Store, Nicalis, Inc. would keep approximately $64,445,313 - that's $8,967,756 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $8,651,634. 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 $51,167,863.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.