I'd say signature campaigns are a very poor goal now, it's hard to earn merit, and even if you will achieve high enough rank, most new campaigns pay very small amounts like $20-50 per week. Most online jobs pay higher than that and don't require such high amounts of time and effort investment to get started.
This is debatable. If a job barrier to entry is low then it means the payments are not that good. Most online jobs that you can easily register are something like copywriting, ads rating, etc which probably pays around $5/hour or lower than that. You need dedication and time to build your portfolio if you want to increase your salary per hour.
I can tell for sure that a $50/week campaign is good enough for someone from my country and can compete directly with 9 to 5 'beginner' office worker's salary here. That being said, you should not see a sig campaign as a target and your 'main job'.
Indeed, well that is the nature of every paying companies or websites in this world, the higher the money you have used for them the for sure, the higher the payment you may receive and the lower the money you have used on them then, the lower the profit you may have, that's how life goes on, but do not worry about that, those things are not necessary if you a right person if you know how to handle situation and if you have an excellent characteristics as a person. All things come by your skills and effort. The good thing on that is the more you used your skills on something, then the more you enhance as a person, the experience is the best for you to become successful in your life.