$100 is the most I would pay for aluminum driveshaft like the FMS one that sold for $150. new back in the day. Now something made by Inland Empire Driveshaft or Dynotech would be much better quality and command more $. I would check anything you buy used to see if it is straight and balanced.
The price skyrocketed several years ago. They are definitely worth it if you change gears. The stockers weren't really balanced for anything over 3.55 gears. At least the ones I have run into. They required re-balancing and it was just easier (at the time) to just get the aluminum one. If you are leaving the stock gears then just leave it. Not worth the coin.