97 Sable Gs Speed Sensor problem

So my speed sensor is going out. Right now when I start the car after it has been sitting for a while, the speed sensor works great, but after driving a little bit the speedometer drops to zero. At this point the shifting on the AT feels a little rough, and sometimes the car gets stuck in 2nd gear (my understanding is that the computer has put the car in fail safe) Luckily this has only happened right as I've been pulling into my apartment parking space. Anyway, I guess I need to get the VSS replaced, does anyone have any idea what this should cost? The transmission has already been rebuilt in January, and I am planning on buy a new car soon.