yahoo62278, you summed things up well. As you can see, some managers already apply an approximate criterion, perhaps someone should analyze their campaigns and make a judgment about how effective such a method is.
However, there was a "rule" that you left out. To establish high-standard conditions, a competitive payment rate must accompany them. Looking further back, the maximum payment rate dropped from $100-$150 per week to an average of $60-$80. Which is more than twice less, including inflation.
The next thing is that campaigns are often short-lived, so it is difficult to expect a highly respectable poster to leave a stable campaign and switch to a better-paid one that will most likely end in a few weeks.