The overwhelming majority of prisoners *will* be let back on the streets. The "contempt" for prisoners, that "lock em up and throw away the key" mentality is stupid. (Sorry Draug...) Our prisons as currently formulated are training grounds to create better criminals, meaning that the prison systems, in effect, do more harm to society than they do good, as they area revolving door - they take in new prisoners while letting out the old - only the old prisoners are now angry (right or wrong) and have received little to no counseling on how to re-integrate in to society, they will have little to no job prospects because of their record, and therefore, even if they manage to stay out, will often become some sort of burden on society - and this is regardless of the crime. Lot's of people sitting in jail for petty drug crimes, failure to pay child support, drunk driving, etc...i.e. not everyone is scum of the earth...and yet they are all lumped together in societies mind.
An alien landing on earth to observe our prison system would be puzzled, for sure, as to why we wish to incarcerate non-dangerous individuals, hold them, and then return them to society in a worse condition than they were abducted. (Of course, the aliens would also want to anally probe them....until they saw what happened in prison...)
As for the cost of housing prisoners...I call complete *bullshit* on the idea that it costs more to house a prisoner than send a kid to school. A quick google search on "prisoner to guard ratio" will show you anything from 5-1 to 55-1 ration -- the truth is probably somewhere in between (I didnt' look extensively), but unless these guards are millionaires, and unless the prisoners are somehow eating more than basic glop (which I doubt), this common assertion can not be true, once a prison building is bought an paid for....but it's a great statement to rile people up without them knowing what you are talking about.
Anyway - the prison systems...and societies view of prisons/prisoners...is a *travesty*.