If LEGO Batman: The Videogame generated the same gross revenue on the Epic Games Store, Traveller's Tales would keep approximately $5,790,295 - that's $1,279,833 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $653,222. 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 $4,510,462.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.