Question: If you are supposed to do UX design estimate, and you need to measure it based on "Simple, Medium and Complex", how do you classify the Simple, Medium and Complex screen design?

What are all the factors that you will consider to define a screen complexity, like number of screen elements, form elements, data structures etc...?

How do we go about it?

Answer: 
It would be the most important to take into account how much the applications or site offers in terms of functionality. Most great UX can be made to look simple or effortless, when in fact these solutions are often the ones that are the hardest to arrive at. Find out from the client exactly what the interface has to do and go from there. There are times when the most complex visual interface will be equal to complexity of the functionality but this is not always the case. Could you provide any more info on the project?
You are right, that is hard to pin down. 

We could create a formula to calculate the complexity of a UX design.

First, lets list all the factors:

1. (F) Functionality, each function in the design multiplies the work
2. (P) Platforms, reworking the UX for a new screensize or platform also multiplies the work
3. (N) The number of elements needed to be considered and displayed

So... Complexity(C) can be calculated: C = P(F + N)