I did a research paper on the Three Sisters volcano complex in Oregon when I was at OSU. One of the things I discovered was how wrong many assumptions about volcanoes were, among them the belief that individual mountains had to be different volcanoes. In fact, the Three Sisters, along with Broken Top, are manifestations of a single magma source.
Once that was known, the single volcano took on a new significance: instead of three separate mountains that might erupt (Broken Top being long dead), this was one monster volcano capable of eruptions of titanic proportions. How titanic? At a rough estimate, sufficient to lay waste to a large portion of Oregon -- but the trouble is that without monitoring, not only won't we know more accurately what such a complex is capable of, but we'll have no clue if catastrophe is coming.
As part of the paper, I had to design a monitoring system. It included tilt meters, hear sensors, seismometers, and other toys involving lasers and GPS. Total cost for a minimal monitoring system sufficient to provide decent warning: $8 million.
That's a volcano in a fairly low-population area, but if it blew at the wrong moment, it could result in 50,000+ deaths -- at a really bad moment, 100,000+, easy.
Then there's the one in downtown Portland that they used to think was dead, but once nearly melted a parking lot. And to the north we have Rainier, definitely still live -- and threatening scenarios in which a million people could die.
There's no way I'm going to join with those complaining about a mere $140 million for volcano monitoring.