I'm an architect, but I've never worked on a prison or anything like that. It's hard to say. I'm sure there's a big price range. You'd probably have to talk to a specialized security consultant to see what the whole system would cost, with controls, etc.
Designing for a 100 year flood isn't really comparable to designing for a mass shooting. 100 year floods, by definition, have a 1% likelihood of occurring each year. That's way, way more likely than a mass shooting. I like the tornado analogy better.
My general point is that people need to think rationally about the trade-offs involved in any decision. (And I'm not saying you aren't.) A lot of people are so traumatized by these outlier events that they want to just DO SOMETHING, even if it eventually turns out to be costly and mostly pointless.