If RWBY: Grimm Eclipse generated the same gross revenue on the Epic Games Store, Rooster Teeth Games, FarBridge would keep approximately $5,411,939 - that's $1,202,442 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $601,628. 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 $4,209,497.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.