Gas Valve: Releases the gas to the pilot flame and the main gas manifold. The average cost to replace a gas valve in a forces air furnace can range from $425 to $675.The gas valves are one of the most important parts of the furnace. No Gas entering the system, no heat. Installation of a new gas valve includes removal of all the wiring connecting to the gas valve.as Valve: Releases the gas to the pilot flame and the main gas manifold. The average cost to replace a gas valve in a forces air furnace can range from $425 to $675.