Move a 3D object on X and Z axis

I have this C# script, it actually can move the object using the mouse:

public void OnMouseDrag(){

     float terrainHeight;
     terrainHeight = Terrain.activeTerrain.SampleHeight (transform.position); 

Vector3 newPos = new Vector3 (Input.mousePosition.x-850, terrainHeight+0.5f, Input.mousePosition.y-300);
transform.position = newPos;

}

I'd like to put the object at mouse position, i think i should use 'screentowolrdposition' but i don't know how.