how much for a "used" or rebuilt front differential, and transmission line?

What is your question?

I recently took my 1999 ML320 in to have the front left axle replaced.I was informed that in the near future I would need a new front differential, and also replace the transmission line. I am trying to get an idea on how much this will cost me. My mechanic says that a new front differential is going to cost $3000. Any advice would be greatly appreciated. Thanks