Banner of Command's base stats are 95.34% gold efficient without the active effect and aura. While the aura is affecting the wielder and no one else, the item is 104.66% gold efficient.
The aura has a value of 270 for every allied champion it affects (including the wielder), giving it a maximum value of 1350 when the entire team is affected and raising the item's gold efficiency to 141.9%.
Upon casting on any of the possible Minions, the selected one gains a permanent Black Shield which grants total immunity to magic damage (similar to Morgana's ability) and Tower Shield (reduced turret damage). All gold gained by the minion goes to the champion who promoted it. Turrets prioritize minions that have been promoted over other minions.
Melee Minion has the most dps of the three and is best used as support for last hitting. Due to the heavy DPS it offers, Melee Minion can easily chunk down the minions health to safely last hit.
Caster Minion having the most range of the three and is best used to offer more aggro damage if the enemy attacks its ally. Despite being the squishiest and having the lowest DPS, the 600 range the Caster Minion offers easily allows it to damage the enemy champions easier and safer.
Siege Minion being the tankiest of them all is best used as a self-pushing minion. Due to the difficulty of destroying the Siege Minion and the heavy attack damage it brings, it can potentially net last hits without even thinking about it as well and last a long time causing it to inevitably reach the enemy turret if left unhindered.