rts building placement

I'm building a RTS type game. So far the game allows selecting objects from a menu of icons, then once selected, lets them place the objects with the mouse cursor by left clicking. There's also a cost for each shape, once the money runs out it says out of money.

One problem im having that I must figure out is, when the icons are clicked, it spawns the object at the same time. Here is my code.

     var cash : int = 2000;
     var boxIcon : Texture2D;
     var sphereIcon : Texture2D;
     var cylinderIcon : Texture2D;
     var cubeCost : int = 400;
     var sphereCost : int = 200;
     var cylinderCost : int = 600;
 
     private var building;
     private var buildingCost : int = 0;
 
 
 
 
     function OnGUI () 
     {
         if (GUI.Button (Rect (10,10, 50, 50), GUIContent( boxIcon, "Build a box") )) 
         {
             building = GameObject.Find("Cube");
             buildingCost = cubeCost;
 
         }
 
        if(GUI.Button (Rect (60,10,50,50), GUIContent( sphereIcon, "Build a sphere") ))
         {
             building = GameObject.Find("Sphere");
             buildingCost = sphereCost;
         }
 
         if(GUI.Button(Rect ( 110,10,50,50), GUIContent( cylinderIcon, "Build a cylinder") ))
         {
             building = GameObject.Find("Cylinder");
             buildingCost = cylinderCost;
         }
     }
 
     function Update () {
 
 
     if(Input.GetMouseButtonDown(0))
     {
 
     var hit : RaycastHit;
     var ray : Ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
     if(Physics.Raycast(ray,hit,300)&&building && cash >= buildingCost)
     {
 
     Instantiate(building,hit.point,Quaternion.identity);
 
 
     switch(building)
     {
         case  GameObject.Find("Sphere"):
         cash -= 200;
         break;
 
         case   GameObject.Find("Cube"):
         cash -= 400;
         break;
 
         case GameObject.Find("Cylinder"):
         cash -= 600;
         break;
 
         default: print ("nothings the case!");
     break;
                   }       
              }
              else
              {
              print("not enough money");
 
              }
            }
         }