It's hard to say with the limited details available whether the ROI is worth it. One thing to note is that the Ukrainians were watching the location, which is 80km behind the front, with a view to hit it with long range fire. In all likelihood, the drone was probably sent there in response to some other sort of intelligence. Based on what they saw, the Ukrainians felt it was worth expending four missiles.
We don't know that they are in the category of "disposable" mobiks; given the location they could be CSS troops.
My own view is that if you can see about 120 troops, that is already more than most Russian companies have. Also, there are probably a bunch more soldiers nearby that you can't see. That would make this a Bn or Bn(-) target, which is probably worth the warheads.
If nothing else, the psychological impact of hitting a grouping of soldiers deep within the AD umbrella and 80km behind the front lines would be considerable.