The problem with mining 3He from the moon is we need an inexpensive and safe system to transport 70 tonnes (assuming that's metric tonnes, that's 70,000 kilograms or 154324 pounds!) of the stuff from the moon to the earth. Granted we only have to do that once every two and a half to three years, but that's a LOT of stuff. The space shuttle could get 24 metric tonnes to low earth orbit, but getting to low earth orbit and getting back to the earth from the moon are two totally different things.
Thankfully, the moon's escape velocity is about a fifth of the earth's, which makes the total delta-v required to actually get from the moon to the earth a LOT lower than the trip from the earth to the moon -- iirc it's something in the range of 40,000 lbs of propellant to bring an Apollo spacecraft off the moon's surface and back home as opposed to six million pounds to get it there.
Yeah.
We're gonna need a cheaper boat.
QUICK EDIT: Let's throw some numbers with dollar signs in there as well. An inflation-adjusted cost for a Saturn V rocket to lift a 100,000 pound (45,000 kg) payload into a translunar injection orbit (translation: you will go to the moon today) is $1,160,000,000 USD. If anyone can find estimates on global annual spending on electricity generation, those numbers would be awesome.
