I did some research about sig campaigns, now i know that most at the time even legendary members are not earning even 50$ per month, some of them say that they earn 100$, but that's also not interesting for me. I personally would not even waste my time to post for sig campaign every day just to get 50$-100$ at the end of the month.
Im not sure where you're getting your information from, but a legendary member can make way more than $50 a month from a signature campaign. Many campaigns pay legendary $100+ a week ($400+ a month) for participating in their signature campaign. I think its worth it for making a few posts a week.