I've been keeping track of how many boxes and runs I've done since the day after the update the nerfed drop rate. So far I'm at:
476 boxes in 674 runs (70.6% drop rate)
44 fortune boxes containing 3 legendaries, 19 epics, 58 rares, 8 commons
106 boxes in 145 runs (73.1% drop rate)
8 fortune boxes containing 1 legendary, 4 epics, 9 rares, 2 commons
I haven't tried farming anywhere else, but from what I've heard the drop rate appears to be the same regardless of what boss or difficulty.
Since fewer boxes are dropping now I've had more excess gold and have started buying more mix boxes with gold than I used to. How much gold you spend on mix boxes depends on how much gold you're getting per day. What I do is try to end the day at about the same amount of gold I started the day at.