One way of looking at oppression and impoverished. Sometimes people join groups because they have no other alternative, either join to survive or die.
Here in the west we think that people have choice but in truth many countries, especially third world countries have to survive by any means necessary
My guess would be at least 100 feet, seems like a lot but you have to figure in the entire bunker and then support posts, cushioning material, radioactive shielding, and extra supply materials for repairs etc. I personally wouldn't be able to make a Blast From the Past style bunker compound, but maybe a nice little man cave out under the barn maybe.