If The Secret of Monkey Island: Special Edition generated the same gross revenue on the Epic Games Store, LucasArts would keep approximately $1,511,621 - that's $404,650 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $69,766. 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 $1,106,971.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.