There was a scene in the hit movie 'Wolf of Wall Street' in which the central character taunted the detective who arrested him, pointing out that even in prison he'd still be a millionaire, whereas the next day the detective would be back on the beat, with holes in his shoes and earning less in a lifetime than a good criminal does in a day. That's true in some cases, but senior detectives and criminal investigators are still making a good living.
Detectives are more than just police officers who have progressed up the ranks. They'll have completed a degree in criminal justice that may take up to four years, and frequently have prior experience in either law enforcement or military positions. Private investigators often work hand in hand with the police - although if you don't have the stomach to inspect a bloody crime scene, you can still make big money investigating fraud or corporate losses.