If Halo Wars: Definitive Edition generated the same gross revenue on the Epic Games Store, Ensemble Studios would keep approximately $3,135,718 - that's $736,851 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $291,234. 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 $2,398,867.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.