If Pillars of Eternity generated the same gross revenue on the Epic Games Store, Obsidian Entertainment would keep approximately $20,721,977 - that's $3,663,474 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,689,361. 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 $16,387,936.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.