$100 a month is achievable with just signature campaign, and if someone do skill works i mean if he sells his skills for bitcoins then he can earn way more then that, and if someone also does trading then he can make a really comfortable living with just bitcoin.
I also have noticed that people in poor countries work about 5 to 6 hours only for less than $100, in some places I am seeing that people work only for $0 to $50 from hours , so if they start to work with bitcoin then they will be able to earn about 4100 only with signature campaign and after that if they get more knowledge then they will be able to earn more money from different sources on internet, and will be richer within a short time period.