how much CPU do codes inside the update function use?

I need some basic but exact information about how calculations are done in unity. I know it depends on the process for example using SendMessage every frame can be a horrible resource hog. but what about setting a value?
I think unity itself runs hundreds or maybe thousands of lines of code to render a frame.
so does it really hurt to have a code in update function? or we should avoid it as much as we can?
I need to know about different calculations so consider this list:

SendMessage, Find, Mathf functions, Setting variables, Writing data on disk, and so on…

A detailed answer would be a great help for many newbies here.

Have no information but, how about you try it? I for myself will not think about that until i come to some trouble that could be caused by that.
Just to a million Sends or finds per Update and you will see the different fps.