If you've got the headliner off, that's the hard part. You've got to be sure that there aren't problems with the roof panel and the tracks, and panel hardware. In other words, what caused the drive assy to break down. Steve or Donnie will have to guesstimate dealer cost parts, etc., for you. I have no idea on that. If you are savvy enough to get the headliner off w/o problems, then you can probably replace the motor-cable assy.
1987 BMW 325