> It makes no sense to get a £80 vet bill on a chicken that cost £20.

This logic is confusing. You are taking a purely transactional view when it comes to the chicken’s health, but you also admitted they don’t turn a profit. In that vein, it makes no sense to get the £20 chicken in the first place.

Your utilitarian view is also the opposite of what the person you’re replying to is describing. Do you believe that if one gets a pet cat or dog for free from the street and they get sick, “it makes no sense to get an £X vet bill on a pet which was free”? And if not, what’s the difference? Neither is making you money.