How Much Am I Paid for a Picture Download from Microstock?
http://microstock.pixelsaway.com/how-much-am-i-paid-for-a-picture-download-from-microstock/

I plotted $/download trends for my top 6 sites.

Are you getting similar values from your portfolio?

It looks eerily close to my numbers....without me actually doing the formal calculations.  The relativeness of your graph is what I feel my RPD is.

SS going slowly up (the highest RPD was in May 12 with 67 cents, it will continue once I turned 38 cents a few days ago, all time rpd 0.49$)
IS down (PP sales are close to 50% the regular income, all time rpd 0.59$ (1494 regular downloads, 3669 pp downloads)
FT stable (pretty much the same during the last 15 months, turned silver in Jan 12, all time rpd 0.72$)
123RF stable (jumping from 60 cents to 90 cents since the start, all time rpd 0.77$)
DT stable (but higher in the latest 3 months, the only agency that had many times over 1$ RPD, all time rpd 0.84$)