If Mount & Blade: Warband generated the same gross revenue on the Epic Games Store, TaleWorlds Entertainment would keep approximately $45,544,571 - that's $7,249,506 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $6,074,260. 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 $36,133,182.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.