The TAM only had 12,000 made, and supposedly 399 went to Apple, so really 11,601. While they didn't sell like hotcakes the one year they were available, compare those low numbers to how many of any Apple product they produce now.
Then add an LCD display for a desktop years ahead of other Apple products, a keyboard with a trackpad on a desktop (plus italian leather palmrest), a kick ass Bose sound system, and it's the first Apple product Jonny Ive designed (previously I read he had designed things like printer trays, etc.), and it's got quite a following.
A regular working TAM that doesn't have the box and all accessories is in the $800-1000+ range, and double that if it does have all of the accessories/boxes. And there were several upgrades possible that also add to the price if included.
Sometimes I've seen some on eBay that are missing the keyboard and Bose subwoofer, which also has the power supply inside, meaning they can't even be turned on and used. Those still sold for $500.