There were a lot of instances of money not wisely spent in the development and production of TMP in addition to necessities (such as new sets needed). TMP also got saddled with a helluva lot of costs for an aborted TV series that had nothing whatsoever to do with the production of the film. If I recall correctly the actual cost of TMP was something around 48-50 million give or take a bit. In that respect that wasn't unreasonable for the kind of film it was meant to be. The inflated figure of about 150 million is with all the aborted Phase II development and production costs factored in.
Sure you don't need the top tier talent to do inserts, but it still takes time and money. And it's one thing to do it on a feature film and something else to be doing a lot of it week after week on a television series.
But we get back to a point I was raising. Mission: Impossible
was bleeding money in production overruns and yet being a popular show cancellation wasn't being seriously considered. They were overspending a lot more than Star Trek,
but people like to say that part of Star Trek's
problem was overspending. Desilu and Paramount weren't happy with what both shows were costing them and yet they were willing to bear it with Mission: Impossible
but not Star Trek.