Mission: Impossible appears to have had a greater tendency for going over budget per episode, and often enough seriously over budget. They were bleeding money to a degree that makes TOS look responsible budget wise.
I've never really seen much of the original Mission: Impossible. What was it that made the show so dang expensive? Was it the cast salaries, or all the location work they were doing or something?
I can't imagine it was the effects.
Geller was a stickler for making things look believable, much like Roddenberry for that matter. It often wasn't enough to make things look
believable (which being science fiction TOS had to do) he wanted it to function believably onscreen. Thats means post-production was fabricating a lot of gadgets for the show. In one instance it wasn't good enough to create the illusion of a three-story elevator he wanted it to actually be three stories to look right.
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.