If you're considering going cheap and not having the disc brakes, there's a Pacific Dualie on Seattle Craigslist right now for $100.
http://seattle.craigslist.org/kit/bik/2556151550.html

And more expensive, but still pretty inexpensive as far as bikes go, there's also a Univega tandem...
http://seattle.craigslist.org/see/bik/2547730599.html