If Stardew Valley generated the same gross revenue on the Epic Games Store, ConcernedApe would keep approximately $173,831,944 - that's $18,911,995 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $23,567,992. 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 $138,179,955.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.