If Tomb Raider: Underworld generated the same gross revenue on the Epic Games Store, Crystal Dynamics would keep approximately $1,500,408 - that's $402,356 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $68,238. 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 $1,098,052.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.