Some of the signature campaigns here are running for more than an year. They are paying about 0.5 BTC in total per month to all participants. Now since they have continued the campaign for so long, they must have analysed the returns they are getting from these campaigns. We may wait to see experience of a  member who has himself used these signature campaigns for promoting his website. Though it can be risky for an online business to share those details in public as the competitors may start using same tactics.