If Watch Dogs®: Legion generated the same gross revenue on the Epic Games Store, Ubisoft Toronto would keep approximately $12,472,417 - that's $2,444,789 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,564,420. 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 $9,825,786.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.