Conqueror is worth 3600 gold.
My last game I played Sion. The Fiora with conqueror was MELTING me even though I was full armor vs a full AD team, so I decided to do some calculations to see just how good Conqueror is on these fighter/bruiser type champions. FYI - These calculations only include auto attack damage numbers that are not increased or decreased by any effect other than conqueror, damage reduction via armor, and Black Cleaver(my control item). I had 3265.64 HP and 233 armor at level 16. This means I would take about 30.03% of all incoming damage (100/100+armor). Fiora had 260.8 AD at level 16. When conqueror was active she would be dealing 115.38 hp per auto, 52.16 true damage per auto and 63.22 physical damage. Calculation - (260.8*.20) + [(260.8*.80).3003], so effectively it took the Fiora 28.30 auto attacks to kill me from full hp. With Black Cleaver instead of conqueror, lets give her 265.8 AD(added 5 to remove conqueror and add black cleaver). Armor would be reduced to 177.08 (would be taking 36.09% incoming physical damage) at max Black Cleaver stacks. Fiora would here be doing about 95.93 damage(265.8.3609) per auto so it would have taken her 34.04 auto attacks to kill me from full hp. Here's why it's really broken, for Fiora to have this AD and have Black Cleaver in this game she would have had to spend another 3000 gold to get black cleaver! It takes 6 less autos to kill me with Conqueror which makes conqueror about 17% more effective than Black Cleaver at the same task. This makes Conqueror worth around 3510 gold on the right champions(due to the stats gained from building tri force second). It allows her and other bruiser/fighters to go Ravenous Hydra -> Tri Force and basically hit their 3 item spike at 2 items. No other keystone is even 10% this powerful on any of their respective users.