Some observations from this great write-up about Magic Find over at Diablofans.
At 0 MF you see a legendary once per 500 elite packs, at 375% after 5 stacks it's more like one per 84 packs. And 25-50% of your legendaries will come from trash, depending on how thoroughly you clear zones.
A few other notes from the article:
- Each item an elite pack drops has about a 0.05% chance to be legendary before MF is applied, and they drop at most 5 tems (with 5 stacks of NV).
- The math thus suggests you get almost exactly 1 legendary per 500 elite pack kills with no MF and no Npehlaem Valor.
- Upping this to 300% base MF, 75% MF from Nephalem Valor, and accounting for the extra NV drop (which can be legendary), the math suggests you will get an average of 1 legendary drop per 84 elite pack kills.
- At max MF, Rough math suggests that if you kill most or all trash mobs, about half of your legendaries will drop from them. If you kill about a third of the trash mobs you see, it'll be more like 25%.
- Because 5 stacks guarantees you 1 rare per elite kill, very high MF causes a MUCH greater increase in the number of legendaries you see over time than rares you see over time. At 5 stacks each rare pack gives 1-5 rares no matter what MF you have, and that minimum of 1 means MF isn't giving you a huge percentage boost in rares seen.
- Another little known fact is that the rare you get from 5-stacks is disproportionately likely to be 4 property, and no more likely to be 5/6 property or legendary than any other drop from an elite monster. Getting an extra drop from the monster means better loot over time, but any time you see just 1 yellow item drop from an elite at 5 stacks it is MUCH more likely to be 4 property than any other yellow item.
- Legendary items are so extremely rare that you will not see many of them no matter what MF you have. You'll also see great variation - while one per 84 packs is much more than one per 500 packs, statistically you'll still occasionally see two back to back or go much longer than the "average" number of elite packs inbetween kills.