I think it would do well for Unreal to weigh in directly on this in the EULA, but in general I think you should act as ethically as possible. If the value of your game is 20 dollars but via Kickstarter magic you’re getting more, you should at least be paying your fee at the base value of the game. If it is less (IE: Get the game today for 10!) then you should bill it at the lower amount - and if you’re really strapped, the lower amount less the fees (so base it on your net income).
Until I hear a direct response from Unreal, that is the method I’d recommend.