The base stats are 85.3% gold efficient, without its passive.
The passive widely varies in cost efficiency based on a champion's base Hp5. As a general rule, champions that build little health will find it less efficient than those that stack health. The passive will make up the remaining value of the item on anyone that finds at least the health to be an efficient stat alone.
This means that Soraka is the only champion to consider Warmog's inefficient at every point in the game.
The living armor protected the greatest troll warrior in the entire realm during the bloodiest and most devastating battles of the Rune Wars. Deep within the dark woods of Crystone the living armor waits to protect its next owner. A word of warning required before seeking out Warmog... it will protect you for a time, but when it grows tired of you, who will protect you from it?
Passive changed to: Permanently gain 4.5 health and 0.15 health regeneration per 5 seconds per minion kill. Champion kills and assists grant 45 health and 1.5 health regeneration per 5 seconds. Bonuses cap at +450 health, and +15 health Regen per 5.