If Risk of Rain Returns generated the same gross revenue on the Epic Games Store, Hopoo Games would keep approximately $8,537,437 - that's $1,841,748 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $1,027,832. 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 $6,695,688.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.