I think it's not possible to support an entire family only on basis of signature campaign earnings from forum, I mean sure that there are high paying campaigns but the cost to sustain lives is much higher, if you see, specially in developing countries, the average middle class earner earns around $1000 a month and still they live not in a very good fashion. In developed countries the cost to survive is much higher, and I guess it's easier to get a job in developed countries that pays way higher than most signature campaigns here. And it would be boring to live solely on signature campaign money because we should work as it's fun, keeps our mental and physical health good and saves us from boredom
You may be right that in develope countries $1500 from signature would not be enough for a family of 5, but even their are countries in some part of Asia, Africa and even some countries in Europe that such amount of money is more than enough to live like a king. Remember this amount of money ($1500) is for 5 members of family that comes in a week, that is in a month each member of the family are having a sum of $1200. In some countries in Africa some government workers are receiving around $300 in a month and they living with it in clothing and feeding. 1500 dollars is reasonable money to live the best life in some countries, you can make your research if you haven't travelled to places .