Assuming you know who's responsible for the campaign.

Any forum rules regarding campaigns - whether it's the OP's "minimum wage" or what you're suggesting - can work only if the forum controls all signatures, i.e. instead of being able to enter custom bbcode users would have to choose from a list of "approved" campaigns and that way you can tie them to managers etc. That obviously creates problems for everyone who just wants a personal unpaid signature.

Perhaps some hybrid solution is possible where signature campaigns have to register with the forum voluntarily and anyone non-compliant can be reported to moderators. Perhaps some clever incentive can be created to encourage compliance. But TBH I can't see that happening given the strong financial incentives to the contrary. If you put any pressure on garbage campaigns they'll just move the management off site.

Absent proper enforcement any good ideas will remain "guidelines" and garbage campaigns will continue to do what they think benefits them most, i.e. spam the forum.