If Planet Coaster generated the same gross revenue on the Epic Games Store, Frontier Developments, Aspyr (Mac) would keep approximately $65,505,246 - that's $9,064,113 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $8,796,170. 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 $52,010,991.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.