Pillage is made up of 2 parts: Accumulation Step, and Salvage Cost Ratio. Salvage Cost Ratio is the amount of the building's cost you "steal" as you deal 100% dmg to it.
For example, having normal pillage (acc step = 1, scr = 0.5) and dealing 50% dmg to a building that costs 100 gold, you'd get 25 gold.
I do believe accumulation step is the amount of gold needed before it'll actually give you something; say you put 5, it'll only add gold once you pillaged enough to get 5 gold.