how much will it cost to replace the Evaporative Emission Vent Valve Solenoid on 2002 Oldsmobile Silhouette

the engine warning light keeps coming on. I can reset it by disconnecting the battery for a while and the car will run for a few days and then the light comes back on. The car runs perfectly even when the light is on. The error code is P0449 GMC

Asked by for the 2002 Oldsmobile Silhouette