If BattleBlock Theater generated the same gross revenue on the Epic Games Store, The Behemoth would keep approximately $22,223,747 - that's $3,885,326 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,894,147. 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 $17,582,526.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.