Cheap construction
Begin building a structure without enough money for completion. Then before construction stops, press R to surrender due to lack of money, but do not complete the surrender command. Wait a few minutes, then select the "Cancel" option. Now the structure will be half-completed.
Note: This only works in versions earlier than v1.22p.

Free minigunners
Use the following procedure to sell the indicated buildings for free minigunners: For the GDI missions, sell a Guard Tower or Advanced Guard Tower. For the NOD missions, sell a SAM site or Obelisk Guard Tower (Obelisk of Light). First, select the "Sell" button in the Sidebar. Then, use the Left Mouse Button to sell the building. Use the Right Mouse Button to activate the selection cursor. Use the Left Mouse Button to select the building and press S.
Note: This only works in versions earlier than v1.22p.