If you are asking what you must pay Epic on a game you sell made with Unreal.
From the agreement.
"pay to Epic 5% of all gross revenue after the first $3,000 per game or application per calendar quarter
(no matter where the money comes in from)
If, on the other hand, you are asking about costs to market for the various platforms…
PC-Windows or Linux. You can market freely.
Either selling through your own website, catalogue, etc, or through a commercial publisher or service such as Steam or Amazon.
XBox and PS - You have to sign up for membership as a Developer before you can deploy for these platforms (you need to demonstrate you are an actual business that produces something of quality)
Android is through Google, there is a $25 US / year fee.
For IOS(iPhone), the Apple Store asks a $99/year developer fee.