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The monetary incentive is usually ALWAYS aimed at an accomplice, they also usually get immunity if they were the lesser evil in the crime. Therefore you can tell when the police believe there are more than one party involved, the reward is much higher.
I think it is sad that people need a money incentive to come forward with important information.
If the police aren't as close as they need to be, a reward is offered to put the final nail in the coffin so they can charge someone. It's just the same if they have very little information, a reward can get the person they need. Other things that are taken into account is circumstances around a missing person or murder, money that is able to be spent by the government and public safety.