Synge created a thread with detailed explanations of the effect of opponent PR on rewards. He explained it better then I ever could.
original thread
My goal here is to show with examples how much effect it has.
These caps are from Winner of Nothing an approximately 1 million pr char with a score high enough to give me a 100% challenge bonus.
These caps are from Maximus the Merciful, a three million pr char who also gives me a 100% challenge bonus
I don't have time to do the math tonight... but here is some real world (heh the irony) data to work with... It looks like the bottom line is that I need a much larger sampling.