Are you an existing UDK subscriber? Asking as you should’ve received an email about this. Here is the snippet from it:
CHOOSE YOUR UDK ROYALTY TERMS
UE4 subscribers can release games commercially by paying 5% of gross revenue from product sales to users (regardless of who collects the revenue). With this model, a game that sells to users on the App Store for $10 would incur a royalty of $0.50 (5% of $10), while you as the developer might receive $7 from Apple (after Apple deducts their 30% distribution fee).
For many developers, the UE4 royalty structure will be less expensive than the UDK royalty structure (which was: no royalty on the first $50,000 of earnings, and $25% of your revenue thereafter).
Therefore, we’re offering UDK developers the choice of paying royalties under either model for products built with UDK. Pick whichever is most favorable to you, and you can apply it to all UDK product revenue from January 1, 2014 onward. Feel free to email email@example.com if you have any questions.