You should not be on forum/make posts on forum just to earn money from signature campaigns. If you post on forum just to make money, your posting quality might degrade or you might end up being spammy without even realizing. Just post to contribute to the forum.

Signature campaign payments differ from campaign to campaign. Your rank also matters. The higher rank you are, the better looking signature you can wear thus paying you higher. Also post quality matters. If you have  low quality posts, you won't get accepted in campaigns.