How much to replace a speedometer sensor on rear axle by a mechanic?

What if there are metal chips on the sensor? I was told there might be some differential issues. What dear god are we looking at there, worse case senario?

I would have it towed, the transmission will not work correctly without the speed sensor. It is somewhat normal to have some metal on the sensor. If you haven't been experiencing any noises or problems, I wouldn't be worried.