When someone gets busted on a big case and it looks like serious prison time is inevitable, and they suddenly get off light its usually because they either become an informant or turned states evidence and agreed to testify. Yes this is in essence buying your way out of trouble but it has nothing to do with how rich you are, just what you know. I've seen the poorest crackheads buy their way out of prison by becoming CI's. And there is no way the public is going to know about the confidential agreement that defendant made with the police or the prosecution. This scenario is about 1000 times more likely than someone bribing everyone involved in the process.
As for Mel Gibson, I don't know what you are talking about. He didn't get out of either of his DWIs. He plead to both of them. The first one was in Canada, which is notoriously light on punishment across the board, and he was just fined. On his second DWI (which was really just his first DWI as far as California was concerned) he was sentenced to 3 years probation with extensive conditions including outpatient treatment. That is not all that light of a sentence at all. That is actually much harsher than the standard offer on a DWI 2nd in all the counties in my area.