If The Elder Scrolls® Online generated the same gross revenue on the Epic Games Store, ZeniMax Online Studios would keep approximately $44,833,729 - that's $7,184,884 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $5,977,327. 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 $35,567,739.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.