The problem here is that there is no widely-accepted definition of what design is in software development. For example, for many people the line between design and coding cannot be drawn sharply, and some see design and coding as being the same thing:
From this point of view, the cost of design is the cost of programming. Other people think "drawing some UML diagrams" or "creating some GUI layout" is design - especially when this task is assigned to some non-programmers. If that's the case, ask those "designers" about their effort, and you will easily find your design cost.
So, first make clear how you define design in your team, and if this task is done in your team somehow "separately from coding". If you cannot separate the design task clearly, don't try to separate in form of costs, that will make not much sense.
EDIT: from your comment, you made clear that you meant UI design (even though you forgot to change the misleading title of your question). Nevertheless, you have to define where you draw the line between "UI design" and "coding". Is UI design for you just "draw some forms with pencil and paper"? Is it the design of the exact behaviour of your UI from the perspective of a user? Is it the design of your inner workings of your forms, for example, when you design your application as an MVP architecture? Based on that (and based on the requirements of your application, of course,) you may come to different conclusions what the percentage of UI design might be in your case.