Well, I paid $2K for mine two years ago with about 180K on it, all stock, so it doesn't sound completely ridiculous to me. It'd really depend on the state of the drivetrain and how much rust. On mine the front U-joints were locked up with rust and the rockers were rusted out, but engine, tranny (it's a 5-speed) and transfer case were all fine, frame and floor was all solid, and interior was good. Paid to have the U-joints replaced, replaced the rockers myself, and did the usual maintenance/tuneup stuff. I'm very happy with it.
(and just an aside, but very cool to see a '60 Seneca on your list, I had a '60 Pioneer years ago)