At startrek.com, there is an article "The Evolution of 'Space Seed,' Part 5" which goes into great detail about the costuming for Ricardo Montalban in that episode.
Star Trek: The Wrath of Khan’s director and writer, Nicholas Meyer, proffers a thesis that art thrives on limitations: that time, budgetary and technological issues, and even censorship may spur a kind of creativity in artists to find ways around these problems and yet still tell their stories or create their designs. Perhaps there is no better example of this in popular culture than Star Trek itself. Filming for “Space Seed” was set to begin on December 15, 1966, based on the 59 7/8 page script written by Gene Coon and Gene Roddenberry and from the inventive premise by Carey Wilber. Before the cameras could roll on the planned 120 scenes, however, several key factors needed to be in place. One of these was the design and creation of the costumes. Interestingly, the character of Khan would have more wardrobe changes than any other male guest star on the original Star Trek, yet the last-minute casting of the superb Ricardo Montalban meant that the costume designers had even more time limitations than normal to make more costumes than usual!
The costume designer, William Ware Theiss, noted “1) I am limited by fabrics and materials. 2) I must try to find design devices which spell out the future for us here and now and 3) I have the same problems of expediency – time and money – that everyone else in the Star Trek company has.”
The article says that Montalban went through 5 costume changes, which is a pretty large number for a single Star Trek episode. Montalban's casting happened in Later November or early December 1966, and filming began on December 15, 1966, so time was very limited.
So although it doesn't address your question directly, there is a strong sense that the budget and time were quite limited for anyone's costumes except Ricardo Montalban - lending support to your supposition that "it's a production economy we're not supposed to notice".