One reason it's hard to answer is that there are so many factors. Different jobs require different skills. Some just need you to be a certain person.
When I was younger, I could make $2,000+ in a long weekend driving a wide area buying cigarettes for various companies. A big part of that was negotiating a good rate and being reliable. Underneath my work was that no one else was going after those jobs.
That is still the essence of how I make my money. I have to do my research and take on work that benefits the scheduler. That doesn't mean that you take crap work. It just also means that no one has an incentive to help you out with specifics.
I will say though, that pushing through a big pile of work for little pay has made me more than able to push out high paying work. It's like any industry. You have to learn the ropes and build your skills so that you can start to raise your rate.