If Beat Saber generated the same gross revenue on the Epic Games Store, Beat Games would keep approximately $33,783,144 - that's $5,592,964 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,470,429. 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 $26,777,501.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.