If LEGO STAR WARS: The Force Awakens generated the same gross revenue on the Epic Games Store, Traveller's Tales, TT Games, Feral Interactive (Mac) would keep approximately $3,179,825 - that's $745,873 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $297,249. 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 $2,433,952.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.