Statistics is hard, you need a large sample; your crap luck is offset by some other SOB that got 3 extra.
Starting from post 121 through 170, I totaled all the runs and talismans from what was posted.
10776 runs gave 145 talismans, for a drop rate of 1.436%.
That includes across all levels, so hard to say if difficulty has an influence, but there are some outliers that said they ran only, or mostly, madness: 340, 18, 5.294%, wow; 645, 20, 3.1%.
Also some outliers the other way: 599, 0; 350, 0. No idea what difficulty those were.
I got my first at about 190, 80-85% on hard, as I wrote this post.
I would say that, as is usually described, higher difficulty gives better rewards.
Sadly, I don't believe I have a team capable of completing madness, so won't get the third purge, let alone enough eclipse.
Bummer; oh well.