Just like the description says... I have spent over 25,000 blood shards and have yet to find a mirror ball and have yet to come across the lvl 70 witching hour... Hell Im still rocking my lvl 60 witching hour because I cant find a belt better than it... Not even the hell cat comes close to the dps output of the witching hour... I have gotten every source in the game multiple times except for the Mirror Ball...
I have yet to even see a mirrorball at all... I have gotten balls of hate, The firebirds eye, chotodos will, the oculus, and more sources I cant remember the names of but have yet to see a single mirrorball.
QFT.
Also see this thread:
http://www.diablofans.com/forums/diablo-iii-class-forums/wizard-the-ancient-repositories/87721-guide-gambling-guide-for-beginners-updated-for-2
About 5500 shards on average to get a +2 Mirrorball.
For a Witching Hour it's much much more because the drop chance is only 0.8% or something like that.