It can be hard to explain why a really simple UI design should cost more than a complex one. Here’s how I like to look at it:
Good UI does the work for you.
Interacting with a messy UI means working hard to figure it out. I think that it is exactly because you are forced to see more and work harder than is necessary that a complex and messy UI could be perceived as having higher value. Maybe in the same way that 10,000 pennies feels like it’s worth more than 100 bucks. But who wants to walk around with 10,000 pennies?
A good UI not only answers your questions, but it also helps you understand what questions you should be asking. It considers its user before it considers its coders. It makes you feel happy and inspired. A good UI does the work for you.