Water heater leaking, shaking, steaming?

I have a natural gas boiler which is used to heat the house and the water. Recently I think the pump has been acting up, and I think it may have just died. A valve near the boiler is leaking, which seems to be the relief valve. The pipes were also shaking violently, and water was turning into steam at some points.... show more I have a natural gas boiler which is used to heat the house and the water. Recently I think the pump has been acting up, and I think it may have just died. A valve near the boiler is leaking, which seems to be the relief valve. The pipes were also shaking violently, and water was turning into steam at some points. I shut the power off and it stopped leaking, no longer shaking or steaming. Now I still have running water but no heating since the power is off.

How much do you think it will cost to repair this, ball-park? If it depends, please give some scenarios and the associated cost.
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