Pre-Mira it was 4 cents I think for witnesses to break even. Since most haven't upgraded to Mira, I'm guessing it's still the same. That's not a point where the chain stops functioning though, plenty of die-hard witnesses here who are willing to pay out of their own pockets for a while to keep this thing going. I mean look at EOS as an example, their break even point is around $4, but the price is lower and it's still going strong regardless.