If FINAL FANTASY XIV Online generated the same gross revenue on the Epic Games Store, Square Enix would keep approximately $33,796,203 - that's $5,594,894 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $4,472,210. 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 $26,787,889.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.