I did some runs through D3 over the past couple days trying to figure out the best way for me to farm. Act 1 is extremely easy to blow through and completely safe, Act 2 is mostly safe and reasonably fast and Act 3 is brutal; I die a lot and killing things is very slow. I am sure Act 2 is my spot for now and I want to figure out how much I should be valuing more Magic Find on gear. My current strategy is to simply clear out the act from start to finish since there isn't much difference in drop quality from mob to mob but ramping up to a 5 stack of Nephalem Valour takes awhile. Given that situation I mostly want to know how MF behaves when I am clearing out packs with a full stack. Right now with a full stack of Valour I have about +200% MF and I get about 2 rares per pack. 1 rare is guaranteed and the other comes randomly and in bursts and droughts.
If I add on 15% more MF by getting a new piece of gear with MF on it how much more loot do I get? I improve my random rare drop rate by (315/300-1)*100=5% and those are about half my rares so 100% MF gives me approximately 2.5% more rares in total. To get 15% MF on gear I generally have to give up another decent mod so I would probably be sacrificing 100 Vitality/Intelligence, 50 Resist All or some equivalent stat. Giving up 100 Intelligence lowers my damage by about 7%, 100 Vitality supplies about 10% of my health and 50 Resist All makes me take 2.5% less damage.
I suspect that 7% more damage is significantly more useful than 2.5% more MF at generating loot because I will not only kill faster but also die less. It is harder to compare Vitality and Resist though because they have low value until not having them gets me killed and then they have a *lot* of value! I do die while running a2 and most of the times I die it is not getting blown away with no chance (where more defensive stats would make little difference) but rather just running out of health and cooldowns to use against particular enemies (defensive stats help this situation significantly). I probably shouldn't underestimate the dps value of being invincible though... if I can't die I can spend more time attacking and looting and less time running and hiding and that is important.
None of this math above takes into account the value of regular magic items. Since magic items can only be worth anything if they are weapons, rings or amulets and are *extremely* unlikely to be worth anything even then I don't think it changes the numbers all that much. The great majority of my valuable finds are going to be good rare pieces and that is what I should focus on maximizing I think. It looks to me like I don't really need more MF gear so much as I need gear with more stuff in general. More damage, health or MF is going to let me get more gear in a useful way and if I get powerful enough I can just jump up to Act 3 and start farming there. My 125% base MF seems like a good place to be at the moment.