If Halo: The Master Chief Collection generated the same gross revenue on the Epic Games Store, 343 Industries would keep approximately $140,906,361 - that's $15,918,760 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $19,078,140. 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 $111,989,151.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.