You can't force people to pay minimums and I don't think that's something the forum should get involved with. It doesn't matter whether someone is getting paid a cent per post or $100. The issue is the quality of posts and campaign managers and regardless of how much a user is being paid they should be upholding some minimum standards and for those that don't there should be repercussions as I've long said. If you let people do what they want then the greedy and lazy will abuse that.
So, I kinda slightly disagree on saying "Signature campaigns are a privilege". It's an opportunity for users to get some financial benefit with the things they already are doing and from the companies perspective - it's a way to monetize the traffic of this forum so that they can build up a business. Both party are getting benefited.
And that's why it's a privilege. Nothing here is guaranteed. theymos could remove signatures anytime he wants if he feels that's what's best.