I'm most familiar with the U.S. system. It's almost like a private prosecution, in a sense. If someone raises an issue of the government being defrauded, they are standing in the feet of the people and usually suing a third-party provider. If they prosecute that up to conclusion and recover money for the government, they get 30%, for example. If the Department of Justice steps in and says it's a valid case and they're going to take it forward, they get somewhere in the nature of 10%.
All reports are that it's a very effective system, and they've recovered billions of dollars in the United States.