

A counterpoint: unmanned technology has developed really fast recently. In old times, one had to be motivated as hell, because taking a shot at a president meant likely death.
In our days, for a technically capable adversary, an attempt costs only moderate amounts, escape is far more likely, and tools can rigged with self-destruction mechanisms to considerably hinder evidence collection.
I’d say that barriers are lower due to drones and robots. Then again, to get drones and robots pointed at oneself, one has to piss off people who have better things to do. That is, people who are unlikely to be desperate, since they could likely emigrate and find a well-paying job in a week.
If I try to predict a profile, I come up with “a technically competent person with an incurable disease, no longer covered by health insurance”. Or perhaps “a war veteran with an incurable disease”.
With a gram of antimatter, you can probably blow a city-sized crater into Earth, so yes, 60 trillion seems like a fair price.
Edit: oh, it’s only 21 kilotons. The stuff is overpriced. :)