If AMF can add about 30 years of healthy human life for $2000 by averting malaria and a human is worth 40x that of a chicken, then we'd need to pay less than $1.67 to avert a year of suffering for a chicken (assuming averting a year of suffering is the same as adding a year of healthy life, which is a messy assumption).

This might be a minor point, but I don't think it's necessarily a given that one year of healthy, average-quality life offsets one year of factory farm-style confinement. If we were only discussing humans, I don't think anyone would consider a year under those conditions to be offset by a healthy year.