Another expense was all the camera repositioning and relighting for the show's numerous quick cutting shots. For example if you just show an explosion or someone removing a ventilation shaft panel it can be over in a few seconds, but if you want to make it interesting you shoot it from several different angles and cut them together in editing to create a more exciting visual sequence. That eats up a lot of time and money. To that end they had two crews working at the same time: one filming the generally conventional scenes and one filming all the inserts to be cut into the main footage in editing.
Ok yeah, that makes sense. Vince Gilligan talks about how long it takes them to shoot those montage sequences on Breaking Bad, with all the different camera setups it requires. So I can only imagine how much more time consuming that kind of thing was back in the 60s.