2004 Pontiac Grand Prix Speedometer Issue

While driving, my speedometer keeps going higher, although I'm not driving faster. When I do come to a stop, the meter hand does go down to 0. However, within seconds after taking off again, my speedometer is at least at 50miles per hour and I've literally just started driving. Do I need to have my speed signal sensor replaced? If so, how much could the parts and labor costs for a Pontiac Grand Prix GT2?