Assuming you are referring to an oil pressure relief valve:
An oil pressure relief valve regulates the oil pressure in an engine by allowing excess oil to bypass the main bearings and flow back into the crankcase. The valve is usually located on the side of the engine block near the oil filter.
Oil pressure relief valves can fail in two ways: they can become stuck in the open position, or they can become stuck in the closed position. If the valve is stuck in the open position, it will cause a loss of oil pressure and will eventually lead to engine failure. If the valve is stuck in the closed position, it will cause excessive build-up of pressure in the engine, which can also lead to engine failure.
The cost of replacing an oil pressure relief valve will vary depending on make and model of vehicle, but it typically ranges from $100-$200.