I had the set plans for cains set drop in nightmare, they are all lvl 22 but still great for exp and mf.ive had other full set plans and single legendary plans drop in hell and inferno. I think it's your mf and some luck the effect your drops.
I tend to just drop the plans for anyone else in my game, if i already know it that is. other than that i save them in my stash for whoever joins. I'm currently paragon level 12 and pretty much just farm anything and everything.