In everything I've ever read about TOS, everyone said they were on a shoestring budget and had to economise on everything.
They had to recycle scenes, voice overs, planet shots, interiors.
The network were complaining to them about the cost, demanding less 'planet' shows as they were too expensive.
Now I suppose they all could have been lying and GR was secretly pocketing the extra cash but I don't think so.
The problem with TOS and all the Star Trek shows was they had to show spacey modern stuff and the prop makers had to build them from scratch. Shows like nuBSG and Firefly show grungy contemporary interiors which are easier and cheaper to source.
I don't know why TOS seemed so expensive in comparison to TNG though, which would have had exactly the same problems. Maybe some TV making costs have decreased over the years?