If Counter-Strike: Source generated the same gross revenue on the Epic Games Store, Valve would keep approximately $26,099,279 - that's $4,457,848 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $3,422,629. 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 $20,665,336.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.