If Bloons TD 6 generated the same gross revenue on the Epic Games Store, Ninja Kiwi would keep approximately $73,023,017 - that's $9,747,547 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $9,821,321. 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 $57,991,037.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.