I don't recommend about the creation of a union for low paying wages. There are campaigns out there that use their personal money to fund their campaign and it is on the discretion of the user if they want to join a low-paying signature campaign or not.
I don't think we need a union for this, as it totally depends on the users if he/she wants to apply for a lower paying campaign or not. It makes no sense to force projects to pay more if they can't afford it.
Like what Hacker said, no one is forced to participate in a low-paying signature. The creation of a union would only bring harm than good as there can never be a "standard" payment in campaign signatures.