Ibis tandems have a cult-like following which says more about Scot Nichol's persona than the tandems. Nichol was and still is someone who marches to the beat of a different drummer and much of that is evident in the tandems that were made during the first life of Ibis, e.g., MORON tubing, hand-job brake bridge, toe-jam pump peg, never mind some of the paint jobs and frame designs. You'll find up tube and double up tube Ibis tandems as well as more conventional frame designs.
At the time they were produced they were at least as "good as" the best tandems being made by anyone else. However, some of them had some funky features that set them apart: some good (pretty good tubing), some not so good (U-Brakes).
So, when considering an Ibis tandem you need to decide if you're buying an icon or simply your first tandem. If it's just your first tandem, then you price it like any other tandem of the same vintage, quality, and componenty. On this type of bike, older or eclectic non-original components will likely be something you may want to replace and that should be factored into your decision process when comparing the total acquisition cost of any tandem to another tandem. A good deal can get awfully expensive if you find that you need a new wheelset and transmission.
If you're looking for an icon then you'd want to make sure that the frame you're looking at actually has some of the iconic chacteristics, e.g., one of the unusual frame designs, the hand-job/toe-jam, or a kick-butt original paint job. While the price shouldn't be any different than what anyone else might pay for a first tandem, the icon status often times causes folks who are looking for an Ibis (or who have one to sell) to pump up the price. On the icon type of bike, the original and very eclectic components are actually more desireable and may help to justify the price of an Ibis vs. a tandem that's become a frankenbike with all kinds of weird, personal upgrades.
So, all of that said, you'd need to decide what the bike is worth to you compared to any other tandem that might meet your needs and make your offer accordingly. I'd probably pay almost $1k for a really nice and iconic Ibis tandem frame in great shape just to hang on the wall... but I wouldn't pay that much for one if I actually planned to ride it.