Bighorner Rancher

Vigilance
{T}: Add an amount of {G} equal to the greatest power among creatures you control.
Sacrifice this creature: You gain life equal to the greatest toughness among other creatures you control.

Fallout

Vigilance
{T}: Add an amount of {G} equal to the greatest power among creatures you control.
Sacrifice Bighorner Rancher: You gain life equal to the greatest toughness among other creatures you control.

Bighorner Rancher’s second ability is a mana ability. It doesn’t use the stack and can’t be responded to. If the greatest power among creatures you control is somehow 0 or less, no mana is added.

The amount of life you gain from Bighorner Rancher’s last ability is determined when the ability resolves. If at that time the greatest toughness among creatures you control is 0 or less (probably because all of your other creatures were destroyed or otherwise left the battlefield in response), you won’t gain any life.