It sounds huge, but to put a different perspective on this, look at the cost of what was lost:
- The Boeing 777-200ER aircraft is worth US$261.5 million[1].
- The average payout for each of the 9/11 victims, including passengers and crew on the planes, was US$1.8 million[2]. We know that MH370 had 239 passengers and crew, so at the same "cost" that would be 239 x 1.8 = US$430.2 million.
So the total value of the MH370 loss is at least US$691.7 million.
Looked at this way, the $100 million spent on the search so far doesn't seem that much to bring closure to the families and to solve the mystery for legal, technical, and future flight safety reasons.
Suppose you lost something very valuable and sentimental, even if you knew it was destroyed, is it unreasonable to spend 15% of its value ($100M/$691.7M = 15%) to find out what happened to it? (Personally, I do think it's worth spending more to keep the search going.)
[1] https://en.wikipedia.org/wiki/Boeing_777
[2] https://en.wikipedia.org/wiki/September_11th_Victim_Compensa...