The cost of running a congregation is fairly low. Being a religion and a nonprofit organization they do not pay taxes. None of the elders or members are paid and any building, or hall maintenance is done with donated labor.
When the utilities bills come up a bit short the brother that does the congregation accounting usually will go up at the end of the service meeting and read the accounting report. If they are a bit short, the money seems to miraculously appear after the accounting report is read from the platform and found to be a bit short.
The watchtower society makes money several ways. They own a tun of real-estate that they buy and sell. They publish books and magazines and sell them at a definite profit. They also have several million older JW's who leave them money in their wills. Many of these older JW's do not have children to leave their money to because when they were younger they had been "encouraged" not to have children because they were too close to the time of the end. The WT society has been named benefactor to many wills. Since they are nonprofit, they do not pay inheritance tax on this money.
Once all those older JW's are all dead and gone I'm sure the JW leaders will find a way to get more money. Their recent attempt at TV advertising should help.