Buying your way out of prison happens because it's about influence and money. Money and influence decide who is considered for a parole hearing, what their decision is likely to be and what the Governor's decision is likely to be. It is that, not your behaviour in prison, that decides your release. It also has to do with the victim's colour. If you are a person of colour and you have killed a white person you not going to get out of prison or off death row for any amount of money.
Money also influences who gets to go to prison or death row.
If you can afford a good lawyer you will not go to prison for so long. Also if you are white and kill a black person you will not go to death row. If you are black and kill a white the jury are very likely to sentence you to death. Dead Man Walking is your life and the events that have happened in it. Now it has been sceenplayed.