So that's an average investment of about 136.000 USD/day since inception. While an inflation rate of 3600 BTC/day corresponds to 792.000 USD/day at todays prices. [ ... ] I was just looking at the plots in your graph though, but I shouldn't be off by that much.
No need to read the graphs for that: the number of coins that they held and bought, day by day, was tabulated by @jl2012. See page 1 of this thread. Only the last plot, the Est. Accum. Investment, is not in the table; but it is easily computed from it.
I was a bit unclear. I was really only reading from the graphs for your "Estimated Accumulated Investment" stat when I wanted to disregard the 2013 data that didn't really seem representative of the current market.
My estimate was that the point for January 1st 2014 had about 34 million in accumulated USD investment. And that should be close enough for my calculations.