It depends hugely on what you are mining. Check out Voisey's Bay figures:

http://www.altiusminerals.com/lab_ni...

Cost of US $1.10 to US $1.15 per pound of nickel. That's how producers make money at $4 nickel.

But we're not lucky enough to be open pit mining a hundred million tonne resource with 2.8% nickel.