How much will cost?

Once you ship your game or application, you pay Epic 5% of gross revenue after the first $3,000 per product per calendar quarter. For a more detailed explanation of how that works, see the EULA and our product release page. We also offer custom license terms to companies who prefer to pay an upfront license fee in order to lower the royalty rate.

You can read more here:

https://www.unrealengine.com/faq