The article has a bit more info.
Spoiler alert: the shock is calibrated to each person to be "painful but not intolerable", and it's about 30 cents a shock for yourself or 60 cents a shock to others.
There may be an initial threshold -- my understanding is that the question would be something like:
"Would you rather be shocked 10 times and get $7 or shocked 20 times and get $9", or "Would you rather be shocked 5 times for $5 or have this chick get shocked 3 times for $4", not necessarily giving a 0 shocks = $0 option.