Cursor goes throgh plane

I'll try to explain the best i can. I have this square plane which is ment to move objects around. But when i move something down it goes trough the plane to like x-100000 y -100000

Code (CSharp):

1.  void OnMouseDown()
2.     {
3.         mZCoord = Camera.main.WorldToScreenPoint(gameObject.transform.position).z;
4.      
5.         x = Camera.main.transform.rotation.x;
6.         y = Camera.main.transform.rotation.y;
7.         z = Camera.main.transform.rotation.z;
8.        
9.  
10.      
11.  
12.     }
13.     private Vector3 GetMouseAsWorldPoint()
14.     {
15.         // Pixel coordinates of mouse (x,y)
16.         Vector3 mousePoint = Input.mousePosition;
17.         // z coordinate of game object on screen
18.         mousePoint.z = mZCoord;
19.         return Camera.main.ScreenToWorldPoint(mousePoint);
20.     }
21.  
22.  
23.  
24.     void OnMouseDrag()
25.     {
26.         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
27.         RaycastHit hit;
28.  
29.         if (!Physics.Raycast(ray, out hit, 100))
30.         {  
31.             position = (transform.position = GetMouseAsWorldPoint());
32.  
33.  
34.             if ((position.x < -4.272f && position.x > -5.272f) && (position.z > -5.21 && position.z < -4.21))
35.             {
36.                 gameObject.transform.position = new Vector3(-4.572f, 0.544f, -4.645f);
37.  
38.             }
39.             else if ((position.x < -3.272f && position.x > -4.272f) && (position.z > -5.21 && position.z < -4.21))
40.             {
41.              (...)
42.         }
43.         else
44.         {
45.             Vector3 mousePos = Input.mousePosition;
46.             gameObject.transform.position = new Vector3 (mousePos.x, 4.43f, mousePos.z);
47.         }
48.     }

The final goal is for the user to snap objects around the plane.

Here is an example:

 

i think for a lack of better solution, i'll make the editing mode with the camera looking downwards, seems to work fine like that..

 

Sorry, i tought it was more related to physics