If Ratchet & Clank: Rift Apart generated the same gross revenue on the Epic Games Store, Insomniac Games would keep approximately $16,870,288 - that's $3,094,474 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,164,130. 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 $13,324,092.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.