If STAR WARS™ Knights of the Old Republic™ generated the same gross revenue on the Epic Games Store, BioWare would keep approximately $4,873,591 - that's $1,092,325 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $528,217. 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 $3,781,265.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.