"Freeway" Rick Ross says there are informants who make as much as $5 million a year. That's an extreme of course, but incomes of $50,000 to $200,000 a year are not unusual. And it's all tax free.
There's another side to it.
In a town near me, the Chief of Detectives was arrested for flat out stealing money from the informant budget. No one was auditing him. He claimed to paying informants and just pocketed the money. He got three well deserved years in prison for that.
Here's some research that Earth First has done on the legions of informants who try to sabotage environmental groups. One of their favorite tricks is to incite people to violence.