Problem with Camera.ScreenToWorldPoint

I am trying to get screen coordinates for a 3D cube. I am using Camera.ScreenToWorldPoint as I don't know any other way to get the screen cords of 3D object in space.

my code:

 #pragma strict
 
 var cubePosition : Vector3;
 var cam : Camera;
 
 function Update () 
 {
     if(Input.GetMouseButtonUp(0))
     {
         cubePosition = cam.ScreenToWorldPoint(this.transform.position);
         //cubePosition.y = Screen.height - (cubePosition.y + 1);
 
         Debug.Log("this.transform.position: "+this.transform.position);
         Debug.Log("cubePosition: "+cubePosition);
         Debug.Log("Input.mousePosition: "+Input.mousePosition);    
     }
 }

• The script is attached to the cube.

• The cube has rigid body with gravity

• cubePosition always returns the same vector(0,1,-10) even if I change the position of the cube.

• If I reverse the y coordinate then it always return (0,604,-10)

• If I click the cube the mouse position is (328,243), I should get this coordinate from my cubePosition if done correctly, right? Well at least that's what I am trying to do.

Maybe its just some stupid little problem that I am not seeing, but I wasted way to much time on this. So I am asking for help or alternative to solve this.