I picked up an early 90s Burley Duet tandem a couple of months ago for a great price, albeit needing some work. (Thank you original owner; I know you'll probably read this.. ) The goal was to strip it down and rebuild it inexpensively, with some updates from my parts collection. The frame had a fair amount of rust starting, the wheels needed a rebuild, but everything was there.
I took it apart down to a bare frame and had the frame stripped and powdercoated in what President Bush would probably call "Nucular Green" My wife picked the color and I knew it was going to be an attention getter, at least!
I didn't like the steel fork and funky double quill stem setup, so I got a new tandem fork from Chuck's Bikes and a modern stem. I reused the cranks and BBs with a set of triple chainrings that were on a 70s Fuji someone gave me. It now has 9 speed Ultegra shifters, a rear XT derailleur and 11-32 cassette.
I stripped the wheels down, rebuilt the front hub, used new nipples and some new spokes, and rebuilt them. I also removed the rear drum; I've kept it but figure we won't need it for the sort of riding we're doing (the MS150 in Sept., for starters).
So, final total investment looks to be around $350 plus some parts from my stash. We've got about 200 miles on it so far and my wife especially loves it. She says she's never going to ride her single bike again.