Celestalon just tweeted this spreadsheet out, it shows the percentage chance that you will be able to get the highest possible iLvl items from each form of content - from normal dungeons to mythic raiding.

https://docs.google.com/spreadsheets/d/1Xe3ZEfKWp2OxX6uDqlf3LQAS1spjuiC5FupVcGJ-M78/edit#gid=0