What Good does Prison do?

Eldin, how expensive is it? o_o

I have a big article on rehab in some magazine in the house, but I'm not willing to go into my attic so I'm just going off old memories here, but-

a bit more than 2 million jailed criminals in the U.S.
a bit more than $20,000 per year per criminal jailed
=
a lot of money

Then, to rehabilitate criminals, you take that base cost and upgrade it- better lodgings (stone walls aren't conducive to "yay i don't hate society anymore" experiences), better food (same), therapist wages, legal scuffles deciding whether a criminal is eligible for rehab or not, etc.

tl;dr: I don't know. More than prison.