Bingo. I bet the one-off single-show models were done as well as required- and no more.
The ones I was referring to specifically were props like phasers, tricorders, etc. that were used throughout the production run, but as you say, no studio wants to spend more money than absolutely necessary. If the prop guys can hack out 10 phasers in a day that will look acceptably on screen, instead of spending a day on each one making them museum-quality, it's not hard to figure out which route the studio will choose.
This is part of why I was so impressed with the Star Wars miniatures. There's detail there that's too fine to show up on even on 4K, and I really respect the obvious pride and effort that went into them.