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.
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