Hi ,
Let me please clarify one point here. If you are doing custom work for hire for your client (i.e., producing a custom app in exchange for non-recoupable work-for-hire fees), no royalty would be due on those work-for-hire fees. You can take a look at Section 5(3) of the EULA.
As bdavis mentioned, if the resulting app is not monetized, then no royalty would be due on that app either.