Back in the 1970s I read a book which explored numerous "behind the scenes" aspects of Star Trek. From what I recall, it contained a claim that it was the most expensive hour on TV at the time. I just finished watching a show about Star Trek which described the original series as a "cheap" show i.e. low-budget. Both can't be right.
Obviously the original series, if expensive, would have been under a lot of pressure to save money wherever possible. Sure, a lot of it looks rather cheesy by today's standards, but is that because of decades of rising production standards along with viewer expectations, or because it really was done on a meager budget? When you consider the ambitions of the show - depictions of aliens, other worlds, and spaceships good enough that you can suspend your disbelief and immerse yourself in the story, it would have pushed 1960s filmmaking to the limit, especially for a weekly show.
Also, its 1960s budget numbers would seem small today because of inflation, so in adjusted dollars, was it really a cheap show, or a premium product for its day?