In everything I've ever read about TOS, everyone said they were on a shoestring budget and had to economise on everything.
TV show economizes.
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.
Go back and read "The Making of Star Trek". The budget was extremely generous
for a one-hour drama. But the production team still had to economise - recycle scenes, voice overs, planet shots, interiors - to bring the scripts to fruition. They were quite transparent about how the money was spent.
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?
Factor in for inflation and the budgets for TNG and TOS were probably very similar. TNG was able to be produced more efficiently because Paramount did things such as pre-selling the international home video and broadcast rights, and projecting the costs for the permanent standing sets across the first five years.
Keep in mind that TOS required two pilots to sell it. The go-ahead for a full season of TNG was given while "Encounter at Farpoint" was still in pre-production. IIRC. Confidence of success was perhaps higher?