If Psychonauts 2 generated the same gross revenue on the Epic Games Store, Double Fine Productions would keep approximately $15,698,568 - that's $2,921,379 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,004,350. 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 $12,392,042.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.