The mafia make most of their money from controlling certain illegal activities (gambling, drinking, prostitution). To do this they must own speakeasies, brothels and underground casinos and all these things cost money. Here's a list of possible institutions the mafia can buy, along with costs and profits:
So a speakeasy costs $5,000 to set up but returns $2,500 every round. "1rd delay" means it takes one round to build. "½rd delay" means that on the first round after it's set up they only take half the profit.
When setting up such an institution you must specify the neighbourhood on the map where it'll be situated. Owning an institution in a neighbourhood means that you get $100 revenue per round from the shopowners in that neighbourhood (protection money). This increases to $200/rd if you employ strongarm in that neighbourhood.
As in the game Go, if you encircle a neighbourhood with neighbourhoods you control then you also gain revenue from it. If you encircle a neighbourhood controlled by an opposing Family then you must either buy it from them or raid it.