I did the giant mines (50 diamonds each) for the past 2 days, 3 mines a day, and I found I lost diamonds. (I think character levels do matter, all of mine are lvl 92)
(yesterday) 1st day I got 23, 32, 36=91 diamonds
(today) 2nd day I got 24, 28, 17=69 diamonds
so for the 300 diamonds I spent in the past 2 days I received 160 diamonds. I lost almost 1/2 the diamonds I had spent.
I'm not doing them ever again, and I would suggest no one else does until they fix the pull rating. I'd like to know if anyone has actually gained diamonds by doing the giant mines.