If OPUS: Echo of Starsong - Full Bloom Edition generated the same gross revenue on the Epic Games Store, SIGONO INC. would keep approximately $4,066,509 - that's $927,241 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $418,160. 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 $3,139,269.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.