If Mass Effect Legendary Edition generated the same gross revenue on the Epic Games Store, BioWare would keep approximately $65,093,086 - that's $9,026,644 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $8,739,966. 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 $51,683,137.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.