If Tom Clancy's Rainbow Six Siege generated the same gross revenue on the Epic Games Store, Ubisoft Montreal would keep approximately $498,046,113 - that's $48,386,010 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $67,779,015. 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 $396,077,590.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.