In today's dollars, Star Trek's budget would come in around a million bucks an episode. And since they had to essentially create a lot of the infrastructure that TNG and the others built upon, they had to make those bucks go a lot further.
That's cheap these days. Some lead actors get paid that much an episode. STNG started off at $1.3 million per ep, but in real dollars, it was only slightly higher than what TOS had. The technology had come so far that it was easier (mind you not easy) to produce a complex regular drama show every week.
RAMA