Assuming a decent campaign manager earns more than the users in the campaign, that would be a dumb thing to do. I also think that if it would happen on a large scale, it would have been discovered once in a while. So it can't be happening a lot.
On this note I wonder why any reputable manager(s) would accept to manage a campaign where the campaign participants earn more than he does that's crazy if you ask me and if it's happening that means the attention of members have not been drawn to such things that's why it has not been exposed.