It could be a test but is there any doubt worldwide that, of all branches of the US military, the US Air Force is the last thing you want to tangle with?
I don't know about stupid. If you can spend X dollars to "force" your opponent to spend X*10 dollars it can be a viable strategy.
I recall the reported costs of "junk rockets" vs those surface-to-air defenses, which I believe are a similar case of the attack being orders of magnitude cheaper than the defense
Sure, that may be the initial ratio, but if it becomes a regular thing, as in sufficiently regular to strain one of our budgets beyond (substituting a few live target shootings for training exercises), we can rapidly develop an appropriate response.
E.g., when there was a new bunker-buster bomb in the initial Iraq war, one was developed in 28 days. Similarly, the MOAB was developed in about a year. There are plenty of teams of highly skilled and well-equipped engineers who will happily undertake to solve that problem.
Also, weapons systems with higher expected rates of use can benefit from economies of scale-ed up production.
Unless they're sending a million balloons this week, it won't be that kind of problem.
No, it doesn't take "far more resources." All pilots, military or civilian, need to accumulate hours to stay current. Flying to investigate or shootdown a balloon isn't going to take more resources than ordinary training would.
The recovery might be something else, but then again, those groups are constantly training anyway, so I'd venture to guess the difference is negligible.