The DATAZ!
I've compared the Historical Market (60 Day Avg) to the Region Market (14 day avg weighted to more recent data), along with some data that I pulled in November 2014. Now I cannot tell exactly when the data was downloaded, but I last edited the original file (was pulling data for another project that never got finished) on November 12th, 2014. Anyway, I've posted the spreadsheet for download and discussion over at The Consortium Forums. The TL;DR is that on average, the percent change in price of Xmog was +3.5%, the overall percent change in respect to every item summed was -6.5%. If you're wondering how these can be different, consider the following:
Item | Old Price | New Price | % Change |
Awesome Xmog | 2 | 4 | 100% |
Cool Xmog | 3 | 3 | 0% |
Neat Xmog | 5 | 3 | -40% |