If Risk of Rain 2 generated the same gross revenue on the Epic Games Store, Hopoo Games would keep approximately $126,079,115 - that's $14,570,829 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $17,056,243. 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 $100,194,750.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.